Got the bottom centering rings in place. After fiberglassing, the tube wasn't quite round down by the fins, so ended up with a pretty significant gap between the rings and the ID of the body tube. Filling that with epoxy as best I can.
Prior to installation, I set the nuts on the metal rods about 0.010" taller than the back of the fins. Put epoxy on the washers, on the motor mount tube OD, and on the back of the fins. Put the rings on and then used nuts on the rods to tighten the plate down to the nuts/washers already on the rods. I'm pretty sure this is not at all the right way to use these rods...I put them in as something of an experiment, and at this point I'm calling it a failed experiment. That's fine, not the end of the world or this rocket.
High quality gauge block setting washer height to just taller than the back of the fins:
Prep for center ring insertion:
Ring in and epoxy everywhere:
Debating what to do with the bottom of the rocket. I wanted the bottom centering rings pushing on the backs of the fins to help carry that load...left that small gap between the ring and fins to allow epoxy to flow into the seam. So, now the motor mount tube protrudes a bit...considering just filling the bottom of the rocket (the bottom where the motor mount tube is protruding, not like the whole fin can) with epoxy (plus glass fibers as a filler since I have a crapton of it) and just fill the whole bottom flush with the bottom of the motor mount and body tube.
From this point, I'm drilling holes in the sides of the body tube above the bottom ring and shooting epoxy onto the top side of the centering ring... Because, you know, Gratuitous Excess. (-: