Anyone mount battery UNDER, or flush in trunk?

I've done some digging, and coming up short on good ideas or pictures.

Anyone ever do a flush mount battery relocation in the trunk that still allows access from the top side?

Im trying to preserve as much trunk space as possible. So far all i can come up with in my head is to weld up a steel battery box, drop it through the floor, brace to frame rail. And fab up a trap door.

Thoughts?

Im already mounting fuel cell under the trunk to save the trunk space, but really wanting to keep trunk 100% usable.

Thanks!!

I had an old Don Hardy , east texas race cars vega , w/ dual battery boxes flush mounted below the floor ,it had an all alum. interior tho. Should be easy to do in a big car , I`d think.