I'd like to know your plan with the Neoprene, Formagasket and aluminum.
You're going to layer them somehow I take it.
I know the Formagasket will stay soft in case you want to remove them.

I cut 8" square access ports to be sealed with 12" sheets of Neoprene,
formagasket, aluminum sheets, and 8x32 screws. The baffels are floor to
cieling, and corners cut at bottom, with 2" holes in the 200 gal tanks.
No 2" holes in the 100 gal tanks. 3 baffels in the 200 gal, and 2 in
the 100 gal. As Keith says look for weldmarks, or take a clothsehanger,
and probe around. I bet you end up cleaning out the tank, so make the
goles big enough to do the dirty deed.

Den 48yf EAGLE