Cessna 150's are nice in that you can open the Rt side window, reach across, pull the door closed, and then shut the window prior to run-up. An "Assist" strap would also be handy on the aft inside of the Rt passenger door. We ask new passengers NOT to close the door because they pull hard on the fragile door handle, stripping the splines on the shaft and bending linkages. Some of these parts are very expensive or "un-obtainable." For straps, check with a store that sells climbing equipment, or a good hardware store. You need high strength strapping or the internal chain type rope. I made mine from a 1" wide military litter strap.
Good luck in your "search" for a solution.