your might consider a rear rack that clamps onto the seat post (or tube the seat post goes into) I used one on my old Schwinn for years and it didn't cost much. Mine was "LeMonde" brand (I think). It may not support as much weight, but there are lots of brands to choose from:

http://www.performancebike.com/shop/...slisearch=true

http://www.nashbar.com/profile.cfm?c...cat%3A%20Racks

http://www.nashbar.com/profile.cfm?c...cat%3A%20Racks

http://www.axiomgear.com/product/racks/seatpost_racks/

-3bikes