I tried my hand at making furnace filter trees; I got the idea from Joey Ricard's article in OGR. I used the stretched furnace filters to apply a lace-like web to a standard Woodland Scenics plastic tree armature. I bent the armature (which comes flat) in a 3D tree-like shape. I then gently pulled the filter material between the "branches" I sprayed everything with Rustoleum Camouflage Earth Brown. When that dried I sprayed on cheap hair spray, and then sprinkled on a blend of Woodland Scenics ground foam (50/50 Medium Green Coarse Turf/Burnt Green Coarse Turf with a bit of Yellow Coarse Turf) I put it under a 150 watt floodlight for a few minutes to dry it quickly. After a few more applications of hairspray and foam, this is what I got:
Many refinements come to mind, such as using real branches, using Noch leaves, trimming the furnace filter to get rid of stray strands, making taller trees, etc. But this was about ten minutes worth of work, and the total cost was too small to be estimated.