YouTube link
Jump to 10:50, and hit pause, to see some finished hills.
Granted, these are stylized with exposed stone for the elevation changes on every hill, they are, in fact, really pleasing to the eye. What is really nice, is how inexpensive and easy they are to make!
They are made using foam floor mats, similar to the Harbor Freight mats I used for my Steading of the Hill Giants base. I painted my mats, on the smooth bottoms, with acrylic craft paint. I've flexed them quite a lot, and after four+ years of usage, no paint has ever lifted, or cracked. I never primed them, either! I just used a wet, natural sponge, wet blending various colors of paint on them. They turned out rather nicely, IMO.
You do not need to use the HF mats, but I would strongly recommend similar mats. The advantages to using the HF mats are threefold: inexpensive for a pack of four large mats; they are extremely durable, and easy to cut and shape; and with mats that are 25" square, you can make some nice, large hills, very quickly! The downside is that they are 0.5" thick.
They are easy to adapt to other styles of terrain, as well: (western style) deserts, alien worlds, etc. Plant some plastic jungle plants, and you could have a little 'Nam action, or your favorite banana republic game terrain. I could see using a base of full-sized mats, painted blue/green, for water, laid down to cover the entire tabletop. Then, overlay that with pieces for dry ground: the exposed water base would be rivers, lakes, etc. There are a lot of possibilities for utilizing these for terrain.
Anyway, take a gander, if interested. If I had seen this video sooner, I might have gone that route. They are ideal for rocky, rough terrain, favored by Dwarves. Cheers!