Jam Sho Goo in there, cover the opening with plastic wrap and tape it tight. After it dries paint it white with acrylic outdoor art paint like you get at Michale's. It helps with the whole spirit of the process if you use duct tape. If the Goo spills over onto the seat just rub your thumb back and forth over it and it peels right off. I once made the back end of my hatch back Subaru out of duct take and slathered the whole thing with Sho Goo. I got rear ended and the hatch and tail light were pushed into the back seat. I used several rolls of duct tape to go around the rear fenders and cover the gap were the hatch was, positioning the tail lights in the hatch window with more duct tape, and slathering the whole mess with Sho Goo for additional reinforcement. I rode around for a few weeks like that, to everyones amusement, until my parts came in at the body shop. The duct tape never came off or needed further attention. If you can make a car this way it should fix a bicycle seat.