If you want to try and change the stabilities of discs, warm/boiling water will work. If you put boiling water in an upside down disc and allow it to flatten, it should get more overstable in most instances. The outside rim should get pulled up toward the flight plate during this process.
If you want to try and add dome, lower the outer edge, and potentially reduce the overstability, you can try finding a pot or other metal object that is slightly smaller than the discs outside diameter. That will let you pour some boiling water into the upside down disc while it's supported, and allow you to put some sort of even pressure on the center of the disc.
This works on base plastic discs okay.