Not sure exactly what your asking for. But, I'll give you something that might work. Bath and Body Works has a car scent line called Scentportables. They sell holders that clip to visors (The discs need a holder.). You can use their scents or you could cut some felt pieces (1/8" thick stuff) to fit inside and put your scent in.
Another possibility is to get some Fresh Wave in their small canister and put it under your seat. Once again, when the contents are used up, put felt or a rag/foam topped with felt (cut to fit) and pour your choice of scent in it.
If it's too strong, you can put some tape (or something similar) on the interior side of the vent lids to slow the transfer rate.