I need to bolt a 50# solar inverter to a stud wall. For three reasons - (1) it needs to be a few inches offset from the studs, so the bolt holes in the inverter won't line up with the studs, (2) I'd feel better if the load were carried by 4 lag bolts (instead of 2), and (3) the inverter needs to sit out 1-2" from the studs, to clear the siding - I intend to lag a short piece of 2x6 or 2x8 to the studs (first removing a couple of siding boards), and then bolt the inverter to the board.

The problem is, how to bolt the inverter to the board. I won't be able to reach the back side of the board once it's attached to the studs. Carriage bolts could work, by installing the into the board before I attach it to the studs; but I'm not satisfied with the amount of bearing surface the head of the carriage bolt would supply. A hex bolt, with washers under the head (on the back side) and under the nut (on the front side), would work. But how to keep the bolt from rotating as I tighten the nut on the front side ?