About 15 years ago, I dropped my HP28S calculator and the case tabs retaining the battery door broke off. I really like this calculator and wanted to keep it functional. The fix that I came up with was to make a tray from 0.030" 5051 aluminum and use the self adhesive hook and loop fastener tape to hold the calculator in place. Due to the pressure from the battery springs, the hook and loop tape sort of flows. About once a year, I have to detach the calculator and re-attach it to maintain the battery door contact. This calculator has been given a quiet life in my lab, so it does not get the bouncing around that it used to, but it is still completely useful.
Dimensions are not critical, the only requirement is that the tray is narrow enough to stay out of the way of the hinge on the spine of the calculator. I chose to make the depth shallow enough so that the edges of the tray are slightly below the top face of the calculator when open. The corners of the tray are welded.