: I want to be able to still haul a trailer, so I'd rather not use
: the tow hitch for a mounting bar, but some other type that would
: fit outside the cap might work.

It is fairly easy to get inside the truck bed and drill a two holes through the cap. After that just bolt on a few 2x4s, or other scrap wood to get the height to match whatever bar arrangement you go with over the cab. You can remove the 2x4s on the cab at anytime and plug the holes with a short bolt and a nut piching together a sandwiceh of 2 metal washers, and a rubber washer.

Since you have a trailer hitch, get about a foot of chain and make a loop out of it. Drop that on the hitch and use it as a connecting point for the tie-down ropes you'll connect ot the back ends of your boats. The front ends of your boats get secured to the front bumper. If you have tow hooks on the front, then use them. Once you have the front and back of the boats fastened so they can't blow off (doesn't need to be tight) you'll just need a rope or strap around the boats. The quick and dirty way is to open the doors, wrap a rope over the boats and pass it through the truck, tie it, then close the doors. The rops is covered by the door gasket.

You really don't need a rack for one or two boats if you can cover an area on the roof with some carpet squares. That protects the roof and boat from getting scratched.
For 3 boats you'll probably want to make "J" frames to hold the outside boats, and carry the third boat in the middle.

consider making the racks from 2x4s first. that will give you an idea of the size. Then worry about mounting them.

If you are towing a powerboat, why not make a rack that attaches to the trailer that the powerboat is on? No drilling or scratching on your truck. You'll have 4 boats on the boat trailer: Three with paddles and one with a motor. A rack would be a good place to put water skis, too. The layout of such a rack would depend on the trailer, of course.

PGJ