Given how light they are I'd just strap them to the side beams of the cargo bed or top tube with voile straps. For really big stuff I put outriggers of a sort on the bed and then strap the lengths down on those (need to be reasonably balanced either side)
I have some curtain rails that need transporting. The longer ones are about 2.5m long
Any tips for transporting them on a Bullitt ?