Our MHP is a wire frame made of wire that has 2'x2' holes. The seed heating mat is hard plastic and provides a solid surface for them to stand on so it doesn't move or fall into the holes. The wire frame is on the bottom, the heat seeding mat sits on top and both of those are inside a pillowcase. I slip stitched the bottom of the pillowcase to the wire at the front and back of the MHP to prevent sagging. Here's the "pics or it didn't happen" for you @aart hehe.
Picture one: View of the underside of the MHP so you can see the lack of sagging pillowcase.
Picture two: On top of the MHP I moved the pillowcase back so you can see the hard plastic heating mat inside and on top of the wire frame.
Picture three: Slip stitching Dr. Frankenstein would be proud of. I had not thought I'd be displaying this when I did it. I stitched it by looping around the wire about 2" in on both ends on of the MHP sat it upright to test and found it held well.
Hope this helps.