If you decide to go the foam route, what I would recommend is Owens Corning Foam Insulation, which is the pink and blue foam commonly used by various modelers. This is a quick example of a cutting I did in about an hour
(the two Dire Avengers are there for scale purposes.)
A non 40k example with some paint on it, is this WIP from a couple prop pieces I built for a show last year
(each was a 4x6 foot prop simulated to look like giant stone tablets handed down from God with the rules of theater engraved upon them and carried by an actor portraying Moses)
Both of these projects are from 1 inch thick Owens Corning Insulation boards that were originally 8x6 foot and sold for 35 at Home Depot. Call around or check website inventories of the hardware stores in your area if you end up being serious about wanting to go this route for hills, in some areas (like here in SoCal) very few stores actually have it in stock.
It isn't hard to use and it looks really nice all painted up, the hard part is of course the cutting, all I had was the good old pocket knife, but I've seen some interesting hot wire cutters that make it a breeze. As Kindred mentioned do NOT use spray paint, as the solvents in the aerosol will not only eat up the foam but release highly toxic fumes.
Hope that helps you out johnboyire!
Peace
~OD