Sheet of rubber cut to fit?

I bought a flyshield for my Savage. It mounted to the fork tubes. The clamps were a "universal" fit and were larger than the fork tubes. The flyshield came with pieces of a firm, rubber-like material from about 1/32 to 1/8 inches thick. You picked the thickness to fill the gap between the clamp and the tube.  The rubber was adhesive backed. You simply stuck the pieces to the inside of the clamp, then mounted the clamp on the fork tube. When I transferred the flyshield to my Kaw Vulcan 800, I peeled off the rubber and stuck thinner pieces on to match the thicker fork tubes of the Vulcan. I think this same method would work for your turn signals. I would imagine you could buy similar material at hobby stores or maybe even Walmart.

Those sound like good solutions.  I certainly have plenty of worthless spent tubes lying around so I'll go that route first.  I'll post back with the final product after I finish.  Thanks.