Your pipe idea might work, but since weight is not too big of a deal, I would suggest getting a steel “open top” drum and putting your rafts in it. They are pretty cheap (I think I paid $40 or so for a 30 gallon open top drum) and available in 15 gallon, 30 gallon, and bigger sizes. I think the 15 gallon size would work for just 2 rafts, the 30 gallon would work fine for rafts plus pfds, etc. Plus - just think how cool it would look strapped to the your boat! In Fairbanks you can them at Arctic Fire & Safety, and you might be able to borrow one from the Fairbanks Alaska Public Lands Information Center (http://alaskacenters.gov/fairbanks.cfm) … not sure where you could find them in the Anchorage bowl…
Good luck!