The man has spoken:
“There was never any hardware in the bags, right back to 2009 when we first ran them. We left the creativity up to the end user, as many have differing views on whether or not to screw into their backbox wood or not. Some mount to weights/blocks, that simply sit on top of the machine. Some use basic L-brackets from Home Depot (a buck each) and some nuts and bolts. Either way, we put two laser-cut holes in it so folks can do their magic as they like. The goal is universal, to vertically stand on top of the backbox.
The piece is not original, and a "bonus piece" we came up with... basically a freebie in each bag, so we didn't go much further with it.
Personally for mine, I did the L-bracket method, parts from Home Depot.”