Well, they beat me to it! Except I am making a metal bed plate that fits a 1st gen Mini. From what I can tell, the OctoGrab is only for the TAZ models. This is long overdue.
I bought a steel PEI sheet to fit a Prusa MK3, bought a cheap pack of magnets, and printed some PLA brackets to mount the magnets along the sides of the Mini bed. See the dwg below. I cut rectangles out of the corners of the MK3 sheet to fit around the corner washers. The brackets have thumb screws to grab the aluminum base plate.
The steel sheet sits on top of the glass and is heated by the glass underneath. I believe this arrangement gives more uniform heat, since the metal spreads the heat out more uniformly than glass, which is an insulator.
I even printed the brackets on the metal sheet by using spring clips to hold the sheet to the glass.
So far, the results have been excellent. I heat the print bed to 70C to get good PLA adhesion.
I was about to get rid of the Mini because it was so hard to remove the prints, especially if they had a large foot print. Now, I can use it with no problems.