A friend of mine built a similar rack (which I hope to reproduce). He wanted to carry 2 kayaks so he made 2 W shaped cradles (one front and one rear - out of carpeted 3/4" plywood). Then for stringers he used a 1" metal pipe between the cradles (with threaded pipe ends and threaded attachements on the cradle). Slightly overlaping the existing roof rack (with grooves cut out of the bottom of the cradle to match rack) provides a stable base. Then, by looping tie down straps over the kayaks and under the roof rack the whole thing is held down firmly. Not the best discription but I can't post a picture, hope it helps.

Mark

> Couldn't you cut the cradles from 1/2 inch plywood and make each
> cradle from two 1/2 inch plywood panels separated by 1x2 (on edge)
> to give 2-3/4 thick cradles shaped to the bottom of the hull. Attaching
> the cradles to the crossbars of your racks is the tricky part. It's
> going to vary dependent upon the rack design. You could put stringers
> between cradles resulting in one cradle assembly that attaches to
> the rack crossbars.

>