The BT-70 tubes are tied together with a 4" long coupler (also from Fliskits). Decided to glass to get some stiffness. Here's the prep for putting a single layer of 4oz glass on the joined tube. I have 10 yards of the 4oz plain weave, this barely used a foot. The OLFA 45mm cutter is a must-have - with a straight edge to guide it, nice clean cuts in the glass fabric every time.
The glassed tube and some other parts. I bought the cluster motor mount from Fliskits as well - but just used the centering rings as templates to cut my own from 1/8" plywood. Stuffer tubes are 9" long to help get good pressure in that somewhat long body tube. Fins are simple trapezoidal, also cut from 1/8" plywood (have a lot of that laying around the shop as well.)
Starting the fin fillets. West Systems 105/205 with enough 406 colloidal silica mixed in to make a creamy paste. Smoothed it out with a plastic spoon dipped in denatured alcohol, used gloved finger also dipped to clean up excess.
One set of fillets had rough spots due to me overdoing it with the colloidal silica mix. Sanded it down as best I could, Bondo and more sanding will take care of the rest. Will sand and similarly touch up the other fillets, but they are much smoother and won't require too much attention.
Bondo sanded down, now on to first primer coat. I use Duplicolor Filler Primer, couple of light coats with sanding in between, maybe some spot filling with more Bondo if I see some rough areas. Creates a stable base for the finish costs.