In case you are new to cutting foam to weird boat angles, just know that it's easy. There are a few methods for cutting, from "worst" to best. Worse just means the cut will be a bit lumpy along the sides. No functional difference though in the end.
1) If it's around 2" or less thick, you can use scissors.
2) Pretty much any thickness you can use a bread knife.
3) An electric turkey knife will make a very nice cut; you just don't have the ideal angle control that you'd have with a professional foam cutting tool.
4) Pro foam cutting saw (this is sort of like a jigsaw married to a band saw, in portable form).
There may be other ways. I have used #1, #2, and #3 and all worked fine. Turkey knife makes the nicest cut by far of the first three methods. If you want to make a cut with just the right angle (and don't have the #4 saw), you can use a Sharpie line on the top, and the edge of a work-table on the bottom (purposely mis-aligned to create the angle). Just don't use a good table without protection.
Likely #1 or #2 will be just fine for a topper though (or you can buy a cheap electric turkey knife pretty easily).
