FIRST, and most important = the term "ROTOmount" is Dick Hanson's motor mount system, and is completely different than my feeble attempt to imitate a Hyde mount.

Now - hansons' retainer system is best explained by him - but - it is a square wood block, approx 1/2" wide, by 3/4" square. Center drilled across the short dimension to clear the head of a 6-32 socket head cap screw. Perpendicular to THAT hole you drill and tap the hardwood block so that a 4-40 screw can be threaded through the block and behind the underside of the head of the 6-32 shcs.
The 6-32 is sticking out of the root rib, where it is anchored in a hardwood block, and glued. Wing goes on, screw goes through hole, 4-40 is tightened down on the screw, preventing the wing from pulling out. Works perfect.
The Pastomount - yes, the balsa discs are just 'in there', faced with 1/64th ply. ONLY the edges of the rubber bands are glued to the ply faces of the mount. Front and rear. That's it.
Works great lasts a long time, cheap to make at home. Is it as good as a Hyde mount? Nope. Close enough for a cheap bas*$#@' d ??? yep.