1. Sink-mount the plate by nesting its round base in a hole cut into the tempera paper-covered mount board. (What is tempera paper, anyway?)
2. Build shadowbox sides the height of the nested plate above the mount board.
3. Lay on a clear acrylic sheet to hold the plate in place.
4. Complete the fitting with a final glazing of acrylic or glass spaced at least 1/8" away from the sheet that retains the plate.
You could also hold the plate in its sink by attaching a sheet of Melinex 516 to the back of a window mat with a round opening slightly larger than the plate, spaced accurately to fit the depth of the plate. In my experience, that is easier than the acrylic method, but any deflection/flexing of the clear film would be visible.
Both of these lean-the-nested-plate-against-a-sheet-of-something-clear methods will probably have to be re-done after expansion/contraction cycles cause abrasion of the retaining sheet.
Glues not only change the condition of the plate, but they have a nasty habit of failing later, too.
Are you sure you don't want to make padded, formed-rod mounts out of fine piano wire?