The first two, with a Space Marine to show scale.
They turned out so well, I made two more. Easy!
I found a pile of 1/2" Styrofoam sheets behind the local Rite-Aid, and snatched it up. I used my band saw to cut the sheet into these shapes, with a 45 degree bevel, glued them together, painted, and flocked. I may paint the rock-surface as I prefer a darker look than this bright tan, but otherwise couldn't be more pleased. The effort-to-reward ratio with these hills couldn't be better.
Update: Decided to paint the surface of the hills. I'm glad I did. They'll need more touch-up now (easy drybrushing) but a layer of black tempura makes them look much more war-torn. Also hides several of my mistakes.