The way in wich I install my boom tubes, is done while constructing the sponsons. Epoxy a block of hardwood, inside of the sponsons, at the places where the booms will be connected to the sponsons. After sponsons are complete, drill a 3/8" hole into the blocks at least 1" into the blocks(the deeper the better) Depending on how deep the holes are in the sponsons, cut your boom supports useing 3/8" aluminum rod, to the lenght that about 1-1/2" is sticking out of the sponson. Useing 3/8" aluminum "tubing" not pipe, the lenght less 1/4" of what you want between the sponsons. On each of the tie end points drill and install 1 #6 bolt. Then drill and bolt the tubing to the tub at the location you want the sponsons to be postion.