I had some spare pipe insulation that fits round a central tube; with a few grooves cut round to push the arrows into. It's like 3D printing but without a computer. I suppose a hot melt glue gun to stick the bits together is very similar, though.
I found it better to find out first, how close to the telescopic tube I could place the arrows without getting the vanes bent. Then I could work out how wide the inner tube should be. I had the vanes alternating top and bottom to give better spacing.