The Rhino-Rack 2500 Series Base Rack System here may do the job - http://www.autoanything.com/roof-racks/61A5774A0A0.aspx

Bit cheaper than the factory job - same manufacturer probably has boat carriers too . . . I don't know how it attaches but your workshop buddy may be able to attach it to the existing roof anchor points.

My previous minivan had no rack: I managed to carry my canoe or kayak by running two rachet straps, one through each of the rear windows, taking a loop around the hull, with tie-downs front and back of course. Foam blocks work without a rack, but I found a set of 4 foam plastic moldings that grip the cockpit coaming - they fit canoes and the plastic kayaks anyway. Nicer and maybe more secure. The high-friction cloth sold for cutlery drawers - also used by woodworkers to stop things sliding around whileplaning etc - help to hold things on a slick roof, it works in the rain.

My present minivan has a rack but other than that there are no convenient places to attach a tie-down. I made a pair of thick aluminum tabs that stick through the front grill about an inch. I found a pair of handy bolts under the hood to attach them to. A hole on each tab allows me to tie a Vee anchor rope to the bow to limit side-to-side movement at the front. A piece of cloth hanging from the stern allows me to check for back end movement through the driving mirror.