Plastic dental trays are great time savers and they allow dental teams to make individual adjustments for better fit.
When you have a tray that doesn't quite fit but you don't want to go up to the next size, you're going to need to make more space. Usually getting adequate space is a problem in the upper posterior portion.
Warm the tray up using a torch. While most of the disposable trays are plastic; they're quite solid, but they do afford you the opportunity to bend them once you warm them, and you can then widen the tray. Pushing up at the angle, while simultaneously pushing out, you'll begin to see the stress on the tray as it reshapes, and it will be molded to your manual specifications.
This allows you to create a wider angle, fill the arch form and still not impinge on the tuberosity. The customization keeps you from moving to the next size up which is often too big to get easily into the mouth.
More comfortable patients make for more comfortable doctors and teams.