Since my last post, I have roughed out and begun finishing the fins. I also grabbed some SharkBite 'PEX' cross-linked polyethylene tubing for the outboard tubes. LPR tubes would be too weak and I didn't have any on hand anyway. The PEX tubing isn't too heavy and it happened to be the right diameter. By 'right', I mean within 3/8" and 3/4" in diameter (it is 3/8").
The attached photo shows what the roughed out fins look like. The tubing has marks along it's length which just happened to be the right width for the fin slots. Rocket Karma, I guess. The fit is nice and snug. I still have to figure out what glue to use on these. I don't so much need to be able to glue to the tube as long as the build up on the fin tabs will keep them from pulling out.