So without sufficient information I picked up a couple sets of cradles from Hobie for the H18 I bought last year. There really are no installation instructions or materials list that comes with them or found really anywhere so I bet there are hundreds of ways to mount these things but this is what we did.
Album of all the pics http://dtbrown89.imgur.com/
I had 8" rollers with a 5/8 shaft.
After receiving the cradles we decided that 1 1/4 PVC fittings would be perfect for the mounting to the cradles as it fittings are just a little larger than the piping it makes for a tight fit without binding in the recess. So to make the "rollers" that allow you to mount the cradles to the 5/8" shaft you need:
1 - 4 Foot stick of 1 1/4" PVC pipe
8 - 1 1/4" PVC end caps
4 - 1 1/4 PVC Couplings
4 - 1/8" cotter pins
1 - 5/8 drill bit
After you get all your things together knock the caps off one side of the rollers to get at the shaft. Then cut the pvc stick into 8 peices about 3 1/2" long.
Drill 5/8 Holes in the end caps and make using 2 end caps, 2 cut pieces of pipe and 1 coupling make a PVC roller. The coupling makes the strap fit almost perfect on the cradles. Install on the cradles and your almost done!
Don't Forget to drill a whole for the cotter pins so the shaft can't fall out.