You've a solar battery bolted to the outside wall, looking a bit exposed, and you know someone handy who could sort a cover for a fraction of the price. Fair enough, a solid job from a mate is a real option. But "solid job" and "the right job" aren't always the same thing, and the gap between them doesn't show up until months later.
What actually goes wrong, even with sloppy work
Condensation builds up inside, and you don't see it until it's done damage. A sealed box with no proper airflow traps warm, damp air against a cold battery casing. That's exactly the setup that causes condensation, and it's not just a bit of dampness. Water inside an enclosure corrodes metal contacts, eats into circuit boards, and dilutes the battery's own internal chemistry, meaning less capacity and a battery that drains faster than it should. None of that shows up on day one. It shows up months later, as a fault nobody can quite explain.
The screws and brackets start to rust. Any bit of untreated metal in a DIY build, hinges, brackets, screws, is sitting there waiting for moisture, oxygen, and time to do their thing. Add salt air if you're anywhere near the coast and it moves faster again. Rust doesn't stay put either, it works its way into whatever it's touching.
Plywood and MDF don't hold up outside, no matter how well you seal the edges. Bare MDF starts swelling within hours once water gets to it, and that swelling doesn't reverse. We're talking a permanent 10 to 20% expansion in the core once it's had a proper soak, edges going soft and fuzzy, screws losing their grip as the fibres around them break down. Even "moisture resistant" grades just slow this down. They don't stop it.
No proper ventilation gap means the battery overheats, not freezes. It sounds backwards, but a sealed-up DIY box with no airflow is often worse than no cover at all on a warm, sunny day. Batteries and inverters generate real heat, and without somewhere for that heat to go, performance drops and, in a lot of cases, it can affect the manufacturer's warranty on the battery itself. A gap between the enclosure and the wall isn't a design nicety, it's doing a job.
A tarp brings its own problems. It flaps in the wind until it doesn't, or it traps moisture underneath just as well as it keeps rain off the top, so you end up with the condensation problem again, just with extra steps and a bit of blue plastic snapping in the wind for the neighbours to see.
And in a proper cold snap, none of this insulates anything. Most lithium batteries stop charging efficiently as they approach freezing. A bit of plywood or a tarp does nothing for that. An insulated enclosure, built for the job, actually holds some warmth in.
"But my mate would actually do a good job"
Fine. Say the build is genuinely well made, solid joints, sealed edges, nothing sloppy about it. Here's what still goes wrong, and none of it comes down to skill.
Ventilation sizing is a spec, not a skill. The airflow gap a battery enclosure needs comes from the battery and inverter manufacturer's own installation guidelines, not general building know-how. It's not something a good tradesperson knows from building sheds. Get the gap wrong and a beautifully built box overheats a battery just as well as a badly built one does.
The material has a ceiling, regardless of the joinery. Marine ply or treated timber, built to a high standard, still lasts years outdoors, not indefinitely. Wood swells and eventually rots because it's wood. Aluminium doesn't carry that ceiling. That's a decision made before the first cut, not a workmanship one.
Warranty compatibility has to be confirmed in writing, and a mate can't do that. A cover that's been through manufacturer testing keeps that protection in place. A homemade build has never been near that testing, however well made. If the battery faults, the manufacturer has grounds to point at the enclosure. How tidy the joints are won't come into it.
Compliance is paperwork, not a finish. Installation has to meet Irish building regs and fire safety requirements, with the ventilation gap maintained as specified in the manufacturer's installation guidelines. A good DIY job still doesn't come with a technical file or risk assessment behind it. That's what protects you if the install is ever questioned, at insurance time, at sale of the house, or by an electrician who won't service a setup they didn't spec.
It's a one-off, not a tested design. Ours has been through repeat installs across actual UK and Irish winters. A mate's build is a first attempt, untested past one weather cycle.
What it actually costs to get wrong
This pattern isn't unique to battery covers. A DIY job that goes wrong almost always ends up costing more to put right than a proper job would have cost in the first place, once someone has to come out and fix what went sideways. It's a familiar story to anyone who's called a tradesperson in after a job's already gone wrong.
Apply that logic here and the maths gets grim fast: a damp-damaged battery, a voided warranty, a call-out, maybe a replacement unit. All to save the cost of a cover that was built to actually do the job in the first place.
No pressure though
If you don't mind carrying that risk yourself, fair play, let your mate have a go, skill was never the issue. We'll still be here when you get tired of checking for damp patches every time it rains, which in this country is often enough.
What a proper cover does differently
Our covers are insulated aluminium enclosures, built specifically to house a solar battery outside, with the ventilation gap engineered in rather than guessed at. They're built to hold up through an Irish winter, coastal salt air included. 
Reviews of the cover read like this: it "looks well made, will insulate the batteries during colder weather," and turns "something that looks quite ugly" into something "very smart." Another said simply: "the final result looks great, the whole system is much tidier now and is protected from the elements."
If something's actually wrong with it, that's what the warranty's there for.
But have a browse Here before you reach for the leftover plywood.
Sources: Silent Killer: How Condensation Shortens Battery Life (panelbuilderus.com); Why Outdoor Electrical Enclosures Rust (eabel.net); What Happens When MDF Gets Wet (virmdf.com); Installing Batteries and Inverters Outside, Grid Neutral (grid-neutral.co.uk)