I used some spare plastic piping I had lying around to put inside the bag so it kept the shape and then slotted it so I could get into the bag. I bent a piece of ali plate around the tubing and then bolted the plate to the 2 holes in the bottom yoke, cut a slot in the back of the bag and slid the plate inside it.
I can try and get some pictures if you want them, but I didn't take any measurements at the time.
It also has the advantage that you can rotate the pipe inside the bag to make it(relatively) weather proof
