I designed a short spring-op coin holder that fits in the cup holder and allows the arm rest to slide over top of it. If you've got access to a 3D printer, you can download the STL files here. It's not necessarily the best solution, because since it's so short, it doesn't really hold a ton of coins (I could only jam in 13 quarters). Space is cut down even more because the coins are held in at an angle to ease the removal of said coin. Anyway, here's a picture of it in my car. I'd love to know if someone else is able to print it and get it to work!