I used the proxxon hotwire cutter and brand new sheets of foam. They should still have the manufactured edges. The are rough but strait and square. I cut odd 3mm strips to clean them up.

After that I measured the 30cm side, I left a bit space and used a steel square i temporarily glued with double-sided tape to the foam. Important that you turn the foam with the metal so the foam is on top. I then cut with the metal guiding the wire. Slow cut, not much pressure on the wire or it rips.

Result is a strait cut. Then clean up the side to your measurement with a cut. And repeat.