I fabricated my own rack out of 2x4's a couple seasons ago. I have not tried to get two canoes on it, but I did get one and a kayak.
My rack drops into the bed. It is self standing and does not attach directly to the truck in any way. This way it is entirely removable. Basically I built two rectangular frames, one for each end of the bed. The bottom edge is a 2x4 that sits flat in the bed. The sides extend about 8" higher than the cab. The tops are mounted about 4" below the tops of the uprights and extend about 4-5" out from them. This makes the uprights act as stops keeping the boat(s) inboard of them. The horizontal supports extend outboard of the uprights to give you something to tie to. These front and rear racks are all permanantly assembled using an wood resin and sheet-rock screws.
You could probably fabricate something to connect directly to the bed or 'sockets' at the corners. I chose not to because I have a tonneau and did not want to have to remove it to use the rack. To support the rack and make it free standing I made two cross baces to interconnect the front and rear parts. These consist of two 2x4's w/ a short section of 2x4 as a spacer between them. I attach one brace to the top of the front brace and the bottom of the rear brace and the other to bottom of the front and top of the rear. One brace attaches outboard while the other attaches inboard, the spacer has a bolt that goes through all three 2x4's, the cross braces and the spacer. It acts as a pivot letting me collapse the braces for storage. The braces are attached w/ bolts using wing-nuts for tool-less breakdown and storage. I normally leave the rack stored in an 8'x8' storage locker in my apartment. Space is tight for me, so having it break down was important.
I actually designed it for use in my old truck ('99 XL Sport). But it fits fine in the new truck too, although I did not use it at all last season (my first w/ the new truck). I actually had it mounted on truck the day I traded. The sales guys gave me all kinds of dirty looks (it is rather ghetto) when I pulled in. But they helped me move it to the new truck. Two people and a minute or so is all it took. As I say, I have not actually used the rack as a rack in the new truck, but it fit fine, so I don't see why it won't.
This summer I plan to use it somemore. It is ugly, and not the best solution. But the price was right. I figure I've got less than $30-40 in wood and hardware in this thing. I can measure and post dimensions and maybe even a diagram or two if you like. Unfortunately I have not photos of it on the truck. I won't have it on the truck for another month or so.
I actually just started another ghetto, uber-cheap rack yesterday. My best friend moved back to the seacost, where we grew up. He's fallen in w/ a couple of our old friends, who are wicked into mountian biking these days. They are trying to get me to come out w/ them. I have a decent bike (hardtail, front suspension) and they say I could probably keep up w/ them. I got talking w/ one guy at a BBQ this weekend. He just picked up a nice full-suspension, $2600 bike. He had it mounted on this rack in the back of his pickup. I decided I needed such a rack, rather than just tossing the bike in the bed, as I usually do. Last night I fabricated such a mount using a pair of Thuttle fork tie downs w/ skewers and a 2x6. I'm about 80% done w/ that. Unfortunately I mounted the tie downs too far outboard and the sprocket of my bike hits the wheel-well when mounted in the right hand slot. I'll have to revisit that at some point. I also want to paint it and maybe have it look a little less fugly..
2003 Ford Ranger FX4 Level II - Silver - Stick - fuelly
2009 VW Jetta SportWagon SEL - Grey - Stick - fuelly
2001 Suzuki Bandit 1200 - Red
Mildly Obsessed With Photography