-- Last Updated: Mar-08-13 2:07 PM EST --
Use a square lashing. For attaching one beam-like structure to another at right angles (PVC tube to crossbar), it'll be more secure than anything you can buy. Choose start and finish knots appropriate to your kind of rope. Two half-hitches will work fine on anything except cheap polyethylene rope, which you shouldn't be using in the first place. Like any other knot appropriate for finishing, this one can be tied with the finishing end of the rope in an already-tight condition. Starting and finishing with a clove hitch would be the traditional method, but that knot MIGHT not stay fully tight. Two clove hitches in series would be pretty bullet-proof though. It's not as complex as it sounds. Total lashing time for the contact point at each crossbar should be less than 90 seconds.