OK, the motor mount didn't perform as well as I'd hoped:
I did get 4 or 5 flights out of it before the prop broke. It was very responsive, at least to me. My son had the longest and flashiest flight on record with the plane shooting straight up, doing a couple of "elevon rolls", and then plunging straight down.
I have a new motor mount put together from some galvanized right angle brackets from Lowe's. I attempted a brief test flight in the back yard, succeeding only in putting two small slices in my thumb - the propeller end is really quite sharp.
Since then it's basically been raining almost every day, or seemingly whenever I have time to fly.
Sooner or later I'll get it flying again - it's ready to go...