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 mounted an Odessey battery up side down to the bottom of the floor pan behind the rear end in a narrowed 65 Corvette. It worked great for 5 years, then I sold it. As far as I know it is still in service.