Agree that won't work. Likely doing bad things to the plastic shell.
I took most foam out of mine, just put the dragbag inside it. Then I have spare space, can put other stuff in as needed, even put a different gun in.
If you are a type that doesn't mess with your guns much (they will stay the same configuration, etc) then time to cut. Get a handier, artsier friend to help if needed. Trace gun onto top foam. Chalk can be brushed off, sharpie is forever. Cut one piece of foam at a time, and all cuts are straight and all the way through. If you want pockets, etc then glue the foam together. There are spray glues that do not destroy foams (many others WILL melt it) and that's how they make foam cases with blind holes like battery compartments. You can, if you want, glue the thick foam to the thin bottom foam when done.
Electric knives will work but leave a slightly rough finish and lots of foam dust IMO. Hot knives are hit and miss; ideally work well but hard to find a good one, can be expensive, and a bit of skill to get used to them (how fast to go, etc).
I like a fillet knife. Get a new cheap one or sharpen and scrupulously clean yours beforehand. Just slice slowly, saw (slowly!) if it gets stuck, etc. Make very gentle curves, OR make a point of cutting only straights and joining.
Plan ahead and practice. When you buy stuff for the next weeks or month, notice the packaging. Any foam thats at all squishy (not styrofoam) and you'd be throwing away otherwise: practice on. Do not just cut, but draw lines and practice cutting specific lines.