ha! sweet!
15inch diameter sidewalls, 8inch inner diameter, 8-9inch wide, 25mm axel. let me know if you want me to send you a sketch.
i was thinking about this dissengaging spool, if you mount the spool on wheel hubs you might be able to use a split pin system through the axel, they would have to be pretty strong and the spool would probably move left+right when the pins are out!!