I have re-designed the 3 piece convertible back panel. It consists of one fixed top panel that holds the input jack and two interchangeable panels, one for open and one for closed.
The two interchangeable panels are attached with stainless steel machine screws (bolts) in to fixed t-nuts. Both panels share the same bolt pattern.
This method allows for one less support brace and eliminates the issue of wood screws stripping out.