How to bolt something to a wood member when can’t access the back side and a carriage bolt won’t do

I need to bolt a 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 2×6 or 2×8 to the studs (first removing a couple of siding boards), and then bolt the inverter to the board.

The unit weighs 50 pounds (23 kilograms.)

The problem is, how can I 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 can I keep the bolt from rotating as I tighten the nut on the front side?