You can buy Yakima "1A" rain gutter clamping towers for 15-50 bucks on ebay.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1846862435

If you don't want to spring for the cross bars, you can substitute pipe of approximately the same diameter, or you can remove the pipe clamps from the top of the clamping towers and use a standard carriage bolt to bolt on wood or whatever you want. The bars are pretty cheap, though, and adjustable.

I looked for a setup like you may want, like you see on work vans with rain gutters and ladder racks, but they all came with cross bars and cost more than used yakima stuff, which is plentiful.

I have a very old (1970's) model Barrecraft ski rack which I still mount each winter to one of my cars. It employes a simple two-piece clamp featuring a angled flange held tight against the frame of the rack with a bolt though it.

The rack itself has "legs which stand in the gutters, then the flangee get tightened against these legs. These flanges incorporate flaps which slide into two slots in the legs to give front-back stability.



------------------
----------------
-----(0)------ bend "ears" 90 degrees inward toward the leg of the rack
------------ (note: put a hole for bolt)
----------
----------
----------
---------- bend bottom to form 90 degree "clamp" under gutter


All you'd need to make the "legs" part of this clamp system would be a flat rectuangular metal piece with a square hole to receive a carriage bolt & wind nut and two slots to receive the "ears" of the flange you'll clamp to it. I figure you can lag bolt the "leg" to the end of a wood or metal cross member of your rack.

You'll need to make very careful measurements for the width of these two crossmembers in order to get the "legs" of the rack to slip into the gutter channels.

Or, you could just shop yard sales for an old ski or utility rack.

Also try: harborfreight.com