Learn from my mistake.... I bought a couple of these kits when they were reissued several years back. What the above poster said, is correct: the fins are only 3 ply plywood. On my particular kits, the plies were oriented so that the root edge to far outer edge, was the weakest orientation of the fins. In other words, the fins were cut in the wrong direction in relation to the plies, they should have been cut turned 90 degrees.
Check your fins. There should be little to no bend of the fin if you hold the root edge in one hand and the far edge in your other hand and try to bend the fin. I'm not confident in 3 ply plywood either, so check your fins for that also. You may have to buy some fin stock and make a new set of fins.
Oh yeah, for blocking the motor tubes... I cut out round pieces of cardboard that fit snugly inside each motor tube and glued them in with epoxy. I used enough 30 minute epoxy in each tube so that there was about .125" of epoxy accumulated above each cardboard "disk".