For secondaries you definitely want to use three pads of RTV to hold them, I've heard of cases using a solid bead that produced warping of the secondary. Under no case would you want to use either a complete layer or a series of beads, and it's not called for by the support requirements. Clean both parts with acetone before gluing up and use fresh aquarium RTV to do the bonding, wait 24 hrs before applying any stress.

I've used PVC pipe for small secondaries (1.5") but my current version calls for a stainless interface to an aluminum structure (see wire spider).

Best,
Mark