Hi, Joe:

Since you are "rolling your own" motor mount, you could just dispense with the ply spacer and make a nosering that fits flush with the fuse from the outset.

Could be a bit tricky getting the right thrust just right, but that's what CAD is for, eh??

For the Caelestia I used the F3AU mount for the Advance, with significantly minimized spacers because it seemed to me that a full-circumference spacer was not necessary when used in concert with a rear mount to further stabilize the motor, especially since it's and inrunner with less rotating mass. About 150 flights thus far and my theory is holding up.