Given that I didn't have foam lying around, I'm building hex hills out of plain old corrugated cardboard.
You can create cutting templates out of these:
https://battletech.nl/assets/hexmaps/grass/hex150.pdfhttps://battletech.nl/assets/hexmaps/grass/hex175.pdfhttps://battletech.nl/assets/hexmaps/grass/hex200.pdfOne slighty advantage of stacking separately cut layers of corrugated cardboard, is that you'll get some "natural" unevenness, which looks rather nice with two layers of alabast filler applied (to hide the corrugated edge).
Keep in mind to offset the corrugation "grain" by 90 degrees between layers for extra strength.
I also glued a layer of 1mm thick "quality" cardboard to the top and bottom for extra strength.
I've painted the entire hex hills with Daler Rowney System 3 Warm Grey first
Then Americana Raw Umber for the flat top area, soon to be followed by some drybrushing.
The rest of the "rockface" will probably get a treatment along these lines:
https://www.youtube.com/watch?v=4FshOmdHwn8I might post some photos when I'm done, if I don't forget...