So I have gotten myself started on a nice box of 'countryside' terrain now. First up are the woods; I cut 60mm rounds from 3mm thick cork tiles and then glued them to 3mm MDF board. Cut to shape with a jigsaw, a coat of sand, a sealing coat of PVA glue, then household paint and flock. The trees are glued in singles and small groups onto 60mm diameter bases made from 2mm MDF (cheap on Ebay)
And there we are. The trees are good and stable, the area terrain is cleanly defined, and they come off easily for storage. The next piece will be a minefield (difficult and dangerous, offering no cover in 40k terms) made from more 3mm MDF and cork tiles, using half press-studs as mines.