So I turned to dynamic duo of wargamer terrain: MDF and white glue.
My first thought was to base the trees the same as I did these ones. I went for the cluster/group option seen below for two reasons. Firstly I wouldn't have had enough of the mdf discs and I didn't feel like getting all handyman and making some more. Secondly I thought that by putting a few on a base I could use them as 'dense' terrain. I'll still use the terrain boards or felt to mark the area of a woods but I can now differentiate between light and heavy woods.
I'll put some more photos up once I've got the bases painted and flocked.