Update: A COMPLETE SUCCESS !

Before I jumped into the freezer/oven technique I also checked my cat can stopper and discovered if I shaved off about 1/8th inch then that would help the two cans to butt against each other at the center of the band.

I cut a few more bands and opened the ends of a few cans. I tossed cans into the freezer and the bands into the oven (at about 350 degrees F). Being impatient the cans were not inside either oven or freezer for more than a couple minutes. Hey, they are aluminum! They are suppose to conduct heat (or lose it) FAST !

I pulled out the band from the oven, then grabbed the can from the freezer and slid the can into the band with very little trouble. I would guess there is about a 10 second window before the metals acclimated enough toward room temperature to make significant movement difficult without the possibility of twisitng the can. I tossed the can/band combo into the oven, let them heat up and then pulled them out and grabbed another can from the freezer. Rinse and repeat BUT I did not rinse with water.

I attached 5 cans in series, bottom-to-bottom and top-to-top. Very tight fits, and with the 4 bands between 5 cans, there were only 2 "pin hole" size openings where the cans buckled inward. So the final product is almost watertight, definitely structurally much more sound, and the oven/freezer angle buys at least 5 to 10 seconds to get the cans joined very well. Success!