Different Types of Sapphire Crystal Glass
Flat sapphire crystal
Watch wearers prefer a flat crystal since it holds distortion. Reading time at any angle is easy because it does not have a fishbowl effect. Moreover, a flat sapphire crystal with a blue AR loses the magnification effect.
A double dome crystal is domed top and underside surfaces. There is no distortion when you look at the watch at any angle. Unlike single domed crystals, it has a zero magnification effect. Add to that, the blue AR on the underside imparts blue hue at any angles - limiting reflection.
Watch modders use a sapphire crystal for watch glass replacement. A sapphire glass does not only look attractive, but it improves vision as well.
Unsure how to do glass replacement? If you need a reference, ask watchmakers and other modders. Or yet, consult professionals.