I have a Ranger. This is what I've done and would do the same with a 150 (and carry a small ladder in the back, I suppose.)
I use an Aluminum cap with side lift windows. Lakeland is the brand. (Found my current one at a boneyard for $80)
For another $80 I got the standard aluminum wrap over the top rack that Lakeland uses. I split the top center support into three equal parts so it doesn't interfere so much with straps, and I slid a piece of 1/8" angle aluminum inside to beef the whole deal up. The angle is held in place with the same self-tapping ss screws that secure the supports to the rack. Instead of mounting the rack to the cap frame with self-tapping screws I through-bolted it with ss screws and nuts.
The rig as is isn't wide enough to carry two full-sized tandem canoes side by side, so I use two 2X4s cut to the combined width of my two widest canoes plus about 3" extra. They really shouldn't be wider than you side mirrors, but other than that there's no reason to cut it too close. I went over the edges of the 2X4s with a larger radius round-over bit and cut 45s on the ends and also rounded those over so if (when) I walk into them I take a glancing blow rather than getting a full-on edge to the forehead from the board.
I routed a groove* to allow the 2X4s to slip over the aluminum rack, cut a couple holes to allow the upright legs of the factory rack poke through the 2X4s, though I was tempted to just cut the legs off. Either way would work. Next I drilled the 2X4s for carriage bolts that hold the 2X4 to the factory rack by steel straps under the factory rack. I epoxied the carriage bolts in this time just for fun.
Then paint the wood to match the truck, or a canoe, or, what the heck, anything that's a nice color, glue some exterior carpet over the top of the boards to protect wood canoe gunwales, bolt it on, throw a couple attachment loops under the hood, and go canoeing. Its about time.
I've done variations on this for my last three pick-ups and think it still can't be beat for the price. I think the attachment system is more secure than Yakima, Thule, Saris, or any of the other currently popular rack systems, and the whole rig stronger assuming the 2X4s aren't full of knot holes, rotting, or anything dumb like that. You can pack camping stuff for the summer in the bed and have it dry and readily accessible in a flash if you want. The rack carries ladders, sheet rock, or furniture as easily as canoes.
Hope this helps.
* I just recalled - when routing out these grooves, its wise to check and make sure your canoe clears the cab roof. You can control the roof clearance by routing deeper on the front or rear as needed. Each canoe is a little different and some with greatly upturned bowlines can be a problem, especially (I would think) on crew cabs.
Wall Mount Boat Racks
Recreational Kayak Paddle
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