caulking or bondo for smoothing
1/2" rabbeting bit on router
paint (unless you want to represent "sono tube") LOL
Really just quick and dirty you could get away with glue + staples to attatch the tube to the side to be flared and the inside of the port and just smooth with caulking, bondo, or your favorite filler. Best would be to rabbet it flush. Really I like the idea of stapleing the tube down after you glue (close staples like stiches in cloth).
Also for the record, my port piece is only 12" deep + the flare which makes 18". I believe the way this worked out with the roundover going down the bottom I have an effective port length of closer to 30 inches judging by the tuning (missed my mark by about 10 cycles too low. It sounds tremendous so I think I'll keep this configuration for a while. I will probably build up my neo motors to replace these when/if I can sell a couple out.
All subs in this thread are for sale and have parts to build up a 15 also if anyone needs (can get parts for 18" with a week's notice).