Or, if you want to remain cheap, have a muffler shop bend a short piece of 2.5" (I think) pipe for you to fit into the hole where the factory box got it's air, and hang your filter from that inside the fender. This is the route I will be taking.

If you have a 3" Air filter, they should be able to flange it to 3 inches for you. Or you can buy a flange somewhere and do it yourself.