In case it'll help anyone I took a photo to show the battery positioning in my AA L85, along with the fuse & MOSFET (it's just a basic MOSFET setup, not my best work but I was in a bit of a rush at the time & it works fine anyway). I'm probably going to add some foam/other padding to help protect the battery and wiring if it gets shaken about, and maybe some velcro to help keep everything in place & out of the way as there are some moving parts nearby.
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The battery is a small pack measuring about 72x26x13mm, rated at 1000mAh 35C (although I have my doubts about that... still, it seems fine so far). There's room to fit a slightly larger pack but it would be more of a squeeze.
On a side note, having the battery placed there does make disassembly & reassembly easier as there's no wiring between the lower & upper receivers to disconnect/reconnect and get in the way
I also found a post from someone else here who did the same with their L85. Not sure which brand theirs is, but it's different to mine:
I knew I'd seen someone else do it this way before