Why wouldn’t chucking two blocks together work? You’d have to be careful that the joint line between the two blocks was exactly centered in the lathe, but once centered I’d think it’d work great. And a lot less wasted time and aluminum.
I think perhaps the best fix for the tensioner plug is to buy a brand new rubber plug and immediately cut the ears off of it so it slips easily into the hole. Then fab a small tab to install under a nearby bolt head so that it just presses lightly on the center of the plug. Even if I were to go aluminum with an O-ring groove, I’d still prefer that retention scheme.