I usually try to do at least two passes with duct tape.
fuck. I had to go fix a hydraulic line at a customer's house. They were just as pissed that it blew as they were their DUCT TAPE and hose clamp fix would not contain the leak.
If you want AC, give it some space. You aren't going to lose much power, as the quick 90* transition will do most of the killing. You'll want some room to wrap it with 1-2 layers of insulation. I have ceramic 1/16" under the 3/32" fiberglass on my DP, and it's possible to do a few highway pulls and touch it. I also have a wrapped downpipe almost touching my pan, and it ends up heating the oil too much on long drives. Not enough of an air gap.
As far as custom pipes, ask your local body shops. They should know of someone that can make them.