This is a great idea as having a foldable bench that can be setup at the range to test different loads would be incredibly valuable. Looking a the picture I am struggling to see how it works.Here is the portable I have made and used often.
The outrigger legs have notches in them as does the middle.
I have a piece of flat steel I welded uprights on that flat steel to hold the plywood secure.
This keeps the legs from kicking out and keeps the middle plywood secured in place.
You could do the same with wood or just build it so it does not fold up.
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