At first I tried making my own, since the store bought ones are awfully expensive for what boils down to sugar, salt and a few spices.
I've had great success with Alton Brown's 8:3:1:1 recipe for rub. 8 parts brown sugar, 3 parts kosher salt, 1 part chili powder, 1 part spice. The parts can be whatever size you want. I used quarter cups, the last quarter being cumin, garlic, black pepper and paprika.
But making your own means hauling around a bunch of spices, and that just wasn't working in my small RV.