I know what you mean. I usually start the day somewhere between 11:00 and 1:00 (depending on the park closing). There's no way I could make it from opening to closing. I plan on visiting again during one of the slow weeks in December (can't wait). Anyway, It will probably be hard to get one that late. I have my own personal
ECV, so this is not an issue for me. But I spoke to someone I met at Epcot one day back in July and they said that
if you rent a scooter from one of the local scooter places in Orlando, it's cheaper and much more convenient that renting one at the parks. They'll even deliver it to your hotel and you keep it the duration for your trip. Google it and see what you come up with. Good luck and hope you have a wonderful visit!