I can's say that I have experienced the foul smells coming from the vents, but it's probably not as humid in Wisconsin. We obviously live in different climates. I seldom use A/C to begin with. If it's a bit warmer out, I prefer driving with the windows open.
I just check my cabin air filter from time to time. Sometimes, I just blow out the dust with an air compressor & re-install it. At times I just replace it. As long as cheaper cabin air filters (under $5) exist, I don't see the need to spend more for them. I also don't see the need to create an alternative.
If you come up with sometime that works really well, however, it would be interesting to see it.