Okay.. that can be a pain..but not bad.
Can you remove the mount by cutting the screw's head off then use plier or a smaller pair of vice grip to get
the screws out.

I'd go with bottom. it's only balsa. Simply becuase it'll look better and the color of white covering is not
as noctible when the color don't match.
Do a good job. when you cut. There's boublers on the sides. Sand it flush later. Make a hatch cover accordingly.
Apply CA to conners to make the wood hard.
Then use lite ply for the conners for the hatch mount on to. Apply CA for the threads.

If you decide to mount the battery pack. In the fuel tank compartment. I suggest making sometype backet or brace so you
can hook rubber bands or tie wraps to prevent the pack from falling.
Or mount the battery on top of the tank
Shove alot of soft foam for a snug fit. Just incase the the hatch comes loose.
Or just CA the threads of the screws , it'll break loose.

Or don't do donuts just for kicks and inspect your model before EVERY FLIGHT.

It won't matter if it's on top or bottom. If the hatch comes loose, you'll just loose it when the model is inverted.lol
Guess how i lost my SE ?