You know, I think you might be able to adapt a walker tote bag to your needs. I believe they are usually tied on or held in place with velcro straps. I don't have one, but google them and I know there are some free patterns out there. Also, some of the board members have posted ones they have made, so someone out there should be able to help.
Edited to add: Here is a site that has a few walker/wheelchair patterns. Either click on the heading "Charity Sewing" or scroll down a little to that section, and then under "walker & wheelchair".