If you have a boat trailer that you want to use as a canoe trailer... You can take your boat off remove the wooden rails for a flat platform to work with I would build a removeable towered T shaped canoe rack and attach it to the frame with U-Bolts.  If you have a friend who is a welder you can use angle iron or tubular steel to build the rack.  My single framd for the back of my truck cost me $28.00 in materials (not counting primer, paint, and eye bolts.  A double is not going to cost that much more.  Angle Iron would be good for that type of rack.  Then your can build a box to go with it and you are set to go!  Then when you finish and get home, convert it back to your boat trailer, store your rack and box until next time...

Bruce