I am considering purchasing one of those racks that go into the hitch receiver and make a T to carry a canoe. Both Yakima and Thule sell them but both seem flimsy. I think that I could build one a lot stronger.
Has anyone built one before? Anybody have plans or directions? Size of tube used?
Find a place that sells steel…
... and look the materials over yourself. You will find two conflicting issues. First, single-point mounting in a reciever hitch of a structure which reaches as high as the roof line WILL be kind of wiggly and perhaps "bendy". Second, any attempt to reduce apparent flimsiness will make it very heavy.
I'd use quarter-inch-walled square tubing to insert into the hitch reciever. Building a good stiff upright support is where you will run into the weight vs "bendy" problem. If you don't mind the weight, go heavy-duty.
You won't be able to stop all side-to-side wiggle, since there will be some play in the reciever hitch. One thing I've never seen done, but which makes good sense, would be to attach (with some diagonal bracing) a crossbar to the upright section so that the crossbar closely matches either the top or bottom edge of the bumper, at least at the bar ends. Install a big bolt on each end of the crossbar which can be cinched down against the bumper (you could rig up some padding on the bolt ends if you wish), and that would end all side-to-side wiggle due to play in the reciever. With a little more complexity, this same setup could be built so that the bolts cinched down against the truck's frame (assuming it is a truck), and that would be much nicer. That's getting pretty elaborate, but it's something you could do if you want something better than the store-bought stuff.
The fact that you are asking for plans makes me wonder if maybe you shouldn't just visit a local welder and see what he can build for you. Someone in that business would be able to "wing it" and come up with something very nice in pretty short order. It would probably cost just as much as a store-bought rack, though.
I have an Extend-A-Truck and it’s not flimsy. My very large husband bounced up and down on it (while it was attached to the hitch, of course) before we loaded up with our two heavy tandems. It’s heavy-duty, easy to set up and take down, and I have nothing but good things to say about it. Great for hauling lumber & etc., also.
For $81.20 on Amazon with free shipping, doesn’t seem worth taking time to make from scratch. (Then again, I don’t weld.)