Given that I didn't have foam lying around, I've built 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).
Make sure to let the filler dry for 48 hours or so between the two layers.
I went for enough cardboard layers, so that each level ended up being a little bit higher than an inch.
Keep in mind to offset the corrugation "grain" by 60-90 degrees between layers for extra strength.
I glued a layer of 1mm thick "quality" cardboard to the top and bottom for extra strength.
I also applied some alabast filler on the top, to create some texture.
I've painted the entire hex hills with Daler Rowney System 3 Warm Grey first, followed with a vertical drybrush using a lighter shade.
Then Americana Raw Umber for the flat top area, followed with a drybrush using a lighter shade.
Finally a few coats of Americana Polyurethane varnish were applied to make it rugged, finished off by applying a few coats of Stuart Sempel's "Coat" ultra-mat varnish to mostly remove the polyurethane gloss.