I think you'd find that you will need to go with a blower type fan that will handle the higher static pressure required to move air through a fairly tight filter. Maybe something like this from eBay:
http://cgi.ebay.com/1-x-Brushless-DC-Cooling-Blower-Fan-9733S-12V-0-2-0-5A-/180390841081?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item2a0021caf9
If this goes away then look up "12v blower fan" on eBay.
You could use a 3M Filtrete filter from Home Depot or Lowes and that would clean some of the stuff from the air, they make them somewhat small. Or poke around your local Wally World and you can usually come up with a replacement HEPA filter for some air cleaner at a reasonable price, I've seen some round Honeywell HEPA filters fairly cheap....they would last a long time in a tear. All you need to do is make a box or baffle and mount the blower to move the air. High air volume is not going to be required. We have a fairly nice portable air cleaner in our apartment and I think it moves maybe 100cfm in the mid-speed that we run it at. But that's for a whole apartment. There's not a lot of air to clean in a TD.
I will probably do the same for our cargo conversion.