My son uses a wheelchair, and I've often adapted bags for use on the back of his chair. Some things to keep in mind-- often it works better to have shorter handles with easy release buckles on the handles-- these can loop over the push handles on the chair. You will probably need a photograph with measurements showing the chair back in order to have a good idea of the size and shape that will work best. Too large a bag will rub against wheels and get scuffed up. I like to use a bag with a flap closure, much like the standard messenger bag style. as that secures items better. I also like to have a small zippered section hidden on the reverse side that keeps valuables and medications secure but within reach.
RST