The core of the build is the fin can. If this is strong and straight, the rest of the build will be easy.
The first step involves bonding the motor retainer and aft centring ring to the motor tube. JB Weld is the traditional choice and there's absolutely no reason not to carry on the tradition by using a generous amount of high-temp to the aft of the motor tube and sliding the aft centring ring to meet the retainer.
Forward Centring Ring
We simply fasten the eye bolt using a washer and nut.
This is a great opportunity to use up some of the inevitable excess JB Weld
Use the fins a guides and tack the mid centring ring in place with CA. This is a point where getting things straight makes a material difference. Time spent will pay dividends later.
Now the fun begins!
Since the mid and aft centring rings are aligned and spaced correctly, the fins can simply get bonded into place. All of the seams are now bonded and allowed to cure.