Square pegs not fitting round holes, what I've done here is pop a similar-sized dowel on the end of the upper spar and secured with a screw before mounting the motor as seen here.
The towel-rail bracket is a tiny bit larger than the dowel, which allows for the smallest amount of gimbaling under thrust. I convene a power-plant meeting including myself with a variety of hats on, and point out that Saturn Five rockets also had nozzles that were not rigidly fixed.
Nonetheless as we're dealing with a few pounds of thrust rather than eleven million, I elect for a compromise and squirt an ample amount of bathroom silicone into each of the housings.... this will allow just a modicum of play as those mighty motors are spun up at 'T minus 3 seconds'.
