2 7/8 drill pipe works for me on a 7' pole. Took a piece of #3 rebar and made a loop at the end. It's at least schedule 40 and maybe more. I just slide it over my hay spike, welded a couple of ears on it and a plate on the top of the fork.
Drilled a 5/16 hole down through both and drop a 1/4x6" bolt through it to keep it from sliding off. The plate on top also stops the rear end of it from lifting up and distributes the load more evenly down the spike. Works great and easy on and off.