You build a brace system, side to side towards each other, run a brace towards the door from each drop towards the rail. Now the door rail can't move. run a brace to connect the two rails. From the center of that brace that goes from rail to rail you run a brace back towards the door as long as you need for the opener. You then brace that rail back to the ceiling as well. Now you mount your opener on that brace. I used 2x2 steel with holes every two inches for this. Kinda kills the whole cathedral reason but oh well.