I plan on renting either an ecv or a wheelchair at the Magic Kingdom and EPCOT. I have some trouble with long periods of sitting. I need to use a cushion for ease of pain. Most wheelchairs accomodate my cushion with no problem. BUT...I really want to rent an ecv so I don't need someone in my group to push me around all day. It will also give me freedom to explore on my own and find a restroom when I am not doing one of the rides with the group. I need to know if the seat on the ecv will fit my cushion or if I need to buy a different one. How big is the ecv seat? Any idea on dimensions?