Moto fly fishing is one of the reasons I built three fly rods. they are four or five piece rods that fit into a 24" rod tube--the 3, 4 and 6 weights. I can easily strap it on the back of the motorcycle. Usually, I strap it down inside the straps of my waterproof Wolfman duffelbag. The rod tube is an aluminum tube with a rubber gasket seal. It's waterproof.
Here's what I've built: