Foam's not a real easy material to work with, tricky to cut evenly and more so to trim it. An electric knife will cut it and shark skin will trim it, though an angle grinder is faster it can damage the surface very quickly if it is allowed to grab the foam. Practice on scraps.

Also for the foam to be waterproof it has to be fairly thick at least a couple inches, usually 3", so the two sides are usually much different in size, making the cutting even more tricky.

Bill H.