A ridged flat fixture is ideal, aluminum or I use corian because I have it in stock. Use a compressible oring that will pull perfectly flat to the fixture with the vacuum on. If your oring stays proud even 1/4 aluminum can bow down in the center. An oilless vacuum pump in good condition with a good fixture will pull 25 inches of mercury give or take a little and should hold your part just fine. If the part does move line the bottom of your fixture with some adhesive backed 120 grit sand paper and that will hold it.