RC Fanatic in post 9 suggests what I would do...use the ball sander in the Dremel. Quick and easy.
If I didn't have a Dremel I'd use the tubing technique, making the tubing short enough to fit in between ribs, and backing up the rib with a small block of wood on the other side so the rib is sandwiched between the tubing and the backup. A bit laborious; I never regretted buying a Dremel.