As far as I know, and maybe some RV-12iS builders will reply, there's not a lot of need for one on the -12iS. You won't even need to cut the canopy - and they do work nicely for that. For the limited usage, get the Harbor Freight one. Or even skip it.
What is handy, though, is a cut-off disk for a Dremel. Those are considerably thinner than die grinder discs, and do an excellent job of separating parts. The Dremel and die grinder disk, along with the sanding drums, are highly recommended. There's a bit of technique to the sanding drums. Use a relatively high speed and very light pressure, taking care to not hold the part against the same spot on the drum. Move the drum axially back and forth just a bit.
Dave