Nice catch. The top panel already works this way but I had not looked in detail at exactly where the screen fitted into the extrusion simply because I was using a standard off the shelf screen. Now I look at what I was planning for the bottom (non-standard size) screen I realise I was intending to fit the screen into the tapped hole slot on the left of your drawing. This made the screen higher up which meant it fouled some other mechanical parts so I looked for a way to mount it to the bottom of the extrusions so it would not do this. But of course I do not need to do this because, as you have kindly pointed out, the necessary feature is already built into the extrusion. All I need to do now is carefully measure and order a new sheet of galvanised steel.
You have elegantly solved an awkward problem for me. I am forever in your debt. Thank you.