I'd just file a radius there now.It';ll be an internal radius, but it'll be better than leaving it as a sharp corner.
Don't use metal for a cover, thats a quick way to a fire if the battery comes loose.
B&Q don't sell anything that'll last for the job (I looked there several times for plastic for stuff like that and they just don't sell anything like it).
You could try polycarbonate, glued together.
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