I would do a couple things differently if I were to do it over.
First you are going to use a full sheet of plywood. No way to get two from a sheet. With this in mind I would make the width 5'-6". This would add another 3 inches to each side.
I would stay with the 35" wide X 33" deep cutout.
Going from 13 inches to 16 inches on the sides will make a huge difference setting up for a shot.
If you are starting with a stand that is down, I would add a pair of metal braces to each side to support the side plywood better. Also the folding front section could stand to be welded solid. I would probably add a couple forward braces under the front plywood also.
The U bolts I am using are 3" wide and flat, not U shaped. They have been very secure.
The front corners of the plywood are rounded. I used a #10 can to mark the radius. Hitting the edges with a router is not a bad plan either for smooth edges.
I really like the Burlap Camo, however it only lasts 2 years when left out in the weather year round. If you could find the burlap fabric on a roll at a fabric shop it would be cheaper I am sure.
Another modification I have thought about is adding a heavy rope around the bottom of the camo fabric. Just wrap the burlap around the rope, and zip tie in place. This would help keep the fabric from blowing around as much.
The first 3 we built were from 1/2" plywood. The last one we used 3/4" plywood. If done again all 4 would be made from 3/4" plywood, it is much stiffer.
My wife made the sand bags from surplus OD Green shelter half's. I have found that air-soft pellets work very well for the fill, and is much lighter weight than sand.
Spray paint is Krylon camo.

Bob R