Thanks for the compliments . This project has been helping me keep my sanity.
Today I made some small wooden cleats out of scrap 1/4" hardwood. After positioning the three layout frames into their proper positions I fixed the cleats inside the corners. This locks them (the frames) securely into place without adding any unwanted holes to the original framework. Once I locate them I drill the through holes with a larger clearance drill which allows me to shift them around slightly for fine tuning. The built-in washer on the screw heads works nicely for this purpose.
The panels can still be removed simply by lifting them up but once dropped over the cleats they all lock together nice and solid with no shifting.
Here's the right hand panel set in place along with the corner elevation and switching tower
Gerry C