The motor mount is pretty straightforward. One gotcha: make sure you put the black motor hook retention ring where instructed or it will drive issues with the TTW fins later as they have a cutout in the root edge of the tab to accommodate the ring. I got mine just a touch too far aft.
Apologies for the picture of the mount drying on a Qualman 24mm rocket stand being out of focus.
The balsa stock in my kit was 3mm thick, but the slots for the through-the-wall fins are sized for 3/32 inch balsa. I have learned today that this is an issue at the balsa supplier which is in the process of being corrected. In any case, I had to widen the slots by about 1/32 inch with a fresh #11 X-acto and a cork-backed steel ruler.
The second and third pictures are before and after fin slots.
Then, the motor mount goes in, aft end of the motor tube flush with the aft end of the body.
Then fins attached. Pardon the fancy fin jig....but when one has such a tool, one should use it
. It was nice to see that my jig and the slots in the tube agreed about the fore/aft alignment.