Another not sure if it would work but why not take 2 oarlocks and mount them to the sides of the boat. Take a piece of 5/4 pressure treated decking and cut it to length. Put a bolt in either end so that it goes down through the oarlock and bolt it down. Mount the downriggers to the board. Easy on and easy off.
The oarlock suggestion sounds like a real good idea. I've read that you shouldn't use pressure treated wood in contact with aluminum, though. Supposedly the salts used in the treating process will corrode aluminum. There are easy ways around this problem, and probably not much, if any, of the wood will actually be in contact with the aluminum anyway, but I just thought that I would mention it.