I did the same. I have the ProMINI slotted battery cover, originally purchased for a different strut brace (which turned out not to fit because of my JCW Sound Kit). I kept the cover, and when I got a GTT brace, it was too wide for the slot. I trimmed the slot wider. If starting out again, I would purchase a regular battery cover from MINI to cut, or cut my original one.Nope, not without cutting a bit more material out. Altough the depth is fine its not wide enough to fit the Aerobox Strut Brace. I bought one a couple of weeks back hoping it would fit, but had to trim it about.
I've trimmed it so there is about 5mm clearance so looks nice and factory fit. I've also bought some black plasticard (1mm thickness) to make a "tray" in the cutout so as to seal the battery lid, but to be honest when the strut is fitted into the cutout you really cannot see the battery at all and would need to be really unlucky to get water in there.