Could be the only reason you smell the little critter is because he's in a nice warm place NEXT TO a heat duct (that's where I'd build a nest) and the only time he really stinks is when the heat duct warms him up real good. Could also be just his little poopies that you're cooking.
Consider a pest inspection. Not a bad idea anyway, just to take care of any bees nests or termites or the like, but chances are, you can locate any rodent dens that way too. Are all of your crawlspace vents properly screened? You don't have any wood stacked up against the house, do you?
Just another thought to add to the list of possibilities...
Good luck. In the mean time, there's always clothespins.