I didn't know about the flex tanks not beign gas compatible til I had a low testosterone moment and read the label...

Here's a couple pics... 1st is a single 20oz tank... thay are about the same height and the 2nd are TWO 20 oz side by side. The two aren't shoved unfrt the wingtube sleeve because I only have that flange cut for a single tank, but shows you if you cut it off all the way acrss they'll slide under there nicely. Add 1/4" foam if you wish and when you velcro or zip down it won't touch the tube at all...

I wouldn't worry about cutting the top part of that former flange til it's flush with the forward floor... lots of interlocking there AND when you epoxy a plate on top let it overhang to catch the floor too and it'll tie it all back in again anyway.

Sounds like the 2-24oz will work too...