Ok ill give you my method… it works perfect if done right!
First get a scarp of foam board and a good straight edge… I just use a strip of 2mm alloy. About 5 inches broad… must be broad!
1, mark the Foamex with a pen or whatever.
2, lay the straight edge in position. “As you would”
3, take a Stanley knife. “Must be very sharp”
4, hold the knife proper! (To do this you must get a good comfy grip of the knife.
Holding the back of the knife low. Low enough that your knuckles are almost touching the foam.)
5, while holding the straight edge. Scratch the surface of the Foamex with the blade
Very gently. Making sure your not digging in or with any kind of pressure. Making sure you stay tight to the straight edge. Remember this is a scratch not a cut!
6, repeat this “scratch” 3-4 times, on the 4th start to apply slight pressure. Then carry on cutting it the normal way you would but always keeping the knife gripped the exact same way and taking your time…
Sounds long and drawn out but works perfect!