Have you ever given any thought to cuting a slot in the back where you would anticipate the main flow to strike and covering the outside of the slot with a piece of ductwork, (Like a forced air heating vent) Then reduce it down to a 4 inch pipe and hooking up the wet/dry shop vac?
I'd use pvc pipe rather than the shop vac hose,(The rings in the hose would trap dust to quickly) And not glue the pvc together so that it can easily be taken a part for cleaning. And remove the main foam filter so it wouldn't gum up, just use replacable prefilters. It should work and be an exhaust and dust collection system in one. Mostly what you get when you spray is dust any way.