Pickup trucks are problematic when carrying canoes. Because the cab sticks up above the bed, either you have 8 feet of canoe hanging out the back or the bow sits on the cab pointing to the sky, catching all sorts of wind and ruining your gas mileage. You need to elevate the back of the boat so it rides level. To do that all you need is a light box frame to sit in the bed of the truck.
Measure from the pickup bed floor to the top of the cab roof. Add 4 inches. Cut four boards to this length for the uprights. Lay a pair of them on the sawhorses. Measure and cut two 2-by-6-inch boards the length of the inside of the pickup bed minus 3 inches. Add a foot to that length and cut two more 2-by-6-inch boards.
Drill and bolt the ends of one of the shorter side boards to the feet of the pair of boards on the sawhorse. If you have a wheel well bump inside the truck, raise the bottom side board high enough to clear the wheel well and bolt it in place. Make sure the two uprights are perpendicular to the side boards and bolt one of the longer side boards to the top edge of the uprights. Leave a foot of the top sideboard extending over the cab past what will be the front leg of the side frame. Repeat with the other side, remember the top board will extend the opposite direction from the first frame when lying on the sawhorses.
Stand up the frames. Measure and cut four boards the width of the pickup bed less an inch or two to give it room. Screw one board to the extended ends of the topside frame boards and one to the flush ends that will be to the back of the frame. This will make a rectangle box frame at the top.
Screw the other two end boards to the bottom of the uprights inside the side boards to make a lower box frame. The 2-by-6-inch construction should provide wide enough joints that no diagonal cross bracing should be necessary.
Wrap the carpet strips over the top of the front and back cross members and staple it in place. Set the frame up in the back of your pickup whenever you're ready to haul your canoe. Screw four eye screws to the sides of the top frame boards, facing outward; one at each corner. The eye screws provide a handy tie-down spot for bungee cords.
Set the canoe on top of the frame. You can safely get two narrow ones up there or a canoe and kayak combination. Attach the end hooks of the ratchet tied downs to corner tie-down points in the bed of the truck. Run the tie-down straps through the top of the frame at the front and back and attach to tie-down points on the opposite side. Tighten the ratchets to pull the straps secure and to hold the canoe rack in place.
Tie down the canoe by hooking bungee cords to the eye screws in the wood frame and passing them over the top of the boat. Attach ratchet tie-down straps to the bow and stern of the canoe, connecting them at either end to the towing loops on the truck frame under the bumpers and at the centre to the bow and stern tie-down loops on the canoe. There is a steel towing loop on each side of the truck frame, front and back where it was attached to the manufacturing line. They work great to make an upside down "V" shaped tie down at the ends. Ratchet the straps tight, a little at a time until you get a balanced pull down on either end of the boat.
You can use 2-by-4-inch boards for this rack and they'll work fine. Just add some extra screws and a light diagonal brace or two.