Anyone here have any advice for making custom boards?

I've got a piece of ply with raised borders on all four sides (internal dimensions 30"*22") and I'm wondering how to decorate it so that it allows for lots of different terrain permutations while also being interesting to look at. So far I've glued the edges of cork board at a few points around the edges. I was planning on gluing down some space marine helmets, skulls, corrugated iron plasticard and other bits and pieces before laying down sand/vallejo mud/crackle paint in various places. I was going to attach the bits/sand first, prime black and then do the paint/crackle paste before doing the rest of the painting. I'm worried about the board looking too flat but at the same time I don't want to make it too uneven or else the terrain may look weird when placed on top.

Here's a crappy phone pic of it in its current state