I can't think of any reason that you couldn't secure a 2" x 4" with screws shot through the trus-joists. I don't understand why you'd have trouble with that. Alternatively, punch a hole through the joist on each side, high enough to allow for a V-shaped strap which goes through the holes, then screws from the other side. PVC isn't very heavy. Strap is a lot cheaper than 2" x 4"s.