Yes Stokie, you are absolutely correct, and I tried that as well. The result was acceptable, but for the time and money I put into it, I concluded that the real units were cheap enough not to waste any time with. As you may note, they do make various lengths, including one that is 20" long, and there's a good reason not to go any longer.
I would advise not to exceed the 2 foot length, as it gets downright frustrating to try and thread tarp through the dogbone any further. The tarp binds up too easily with longer lengths and the dogbone bends too much under load. I made mine out of Sched 80 PVC and dowel, and ended up cutting it in half to make it more servicable, and even so I still preferred the smaller ones. Better just to use three or four smaller ones and cross hitch them to a couple stakes outrigged wider than the length between the grabbits. Cross hitching them negates any load warp on the bind, meaning the load on the tarp is laterally balanced. You get that with any sort of tie down system.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)