I agree that the corners should be filled in with the resulting mirror. The white would easily show on the corners. Also it would be fantastic of there were layout lines placed, where the folds would be and also marking the center of the width and length. It would help with placement on the stretchers. I guess the inputs for the GUI would be
the actual width of the stretcher,
the actual width of the stretcher
Actual depth of the stretcher
bleed that would be added to the actual depth of the stretcher or a note to add to the stretcher depth.
checkbox(s) with the placement guides.
based on the above Q would automagically interpolate up or down to the size entered, crop to fit the aspect ratio and print
This would save me a ton of time getting canvases prepped to print. If you would like a sample of what the end result that I have been getting with a PS action I would be happy to send it to you.