You could use the same clips that hold the 5x4 back on with some recesses in the extension. As the 5x7 frame is rectangular, you don't need the very corners of the opening as image area.

That would retain the ability to rotate it.

I would start by making a simple square plate (with light trap recess if that's the way Wisner/Zone VI do it) which clips onto the back in place of its normal back and build it up from there.