From the people who first brought you sapphire inserts:
Made from pure optical grade sapphire, and highy polished.
29.7mm outer diameter (some 35mm bangers with 2-2.5mm walls will have shrinkage or imperfections blocking them from perfect fits - please measure if you are unsure)
2.5mm wall thickness
Careful tapers at every edge, rounded junction of wall/dish for easy cleaning.