That makes sense,do you think the box will do the same job? It's 18mm ply which I glued and screwed with a flexible glue,it fits snugly in the frame and was going to tap screw it on, it's just held on with the yellow clamp to test
If you screw it on properly it should be plenty stiff.
Then you just need a little bit of horizontal support behind the box. Like in your own sketch and my second one to avoid sideways flex.
That makes sense,do you think the box will do the same job? It's 18mm ply which I glued and screwed with a flexible glue,it fits snugly in the frame and was going to tap screw it on, it's just held on with the yellow clamp to test