This is my V2 belt tensioner tool. I'll add a photo or separate post for V1.
This one is made from a one-size-fits-none "universal" belt tensioner tool; nuts and seat bracket from an E46 3 series; two stainless steel bolts; and a PowerKraft socket. I blued it just because I've never used bluing before. I like it a lot.
The curved seat bracket rests on the tensioner pivot mount, and the pins mate fit into holes on the tensioner pulley. A ratchet or breaker bar fits into the socket to lever the tensioner up. The pre-fab units are one piece, so take up a lot of space. This one takes up very little space, since there is no permanent handle. Plus, it was free.