Traditionally, the trampoline mod entails buying a bunch of 2mm silicone balls and inserting one into the center hole in the base of each switch. This has the effect of shortening and quieting the downward stroke, but makes bottoming out feel extremely squishy. I'm a very heavy typer, so bottoming out is a constant for me, and squishy keys is a thing I really don't like.

The solution was to buy a thin sheet of leather, then use a hammer and a small hole punch (the 2mm size made for a good fit for my switches) to cut cylinders out of it and push them into the center hole of my switches.

The keys now have a pleasantly short stroke, are slightly quieter than without any mod, and the landing is softer without feeling squishy.

I suggest trying this mod on an expendable switch before doing a whole board:

1. It can be very difficult or impossible to remove the material once inserted in this way. (I was able to do it with an open safety pin.)
2. It can mess with switch activation if the material is too thick. To test for this, insert the modded switch into a live keyboard, then press down on the key from different angles. (The activation distance can be different depending on the angle.) Make sure that you consistently get a single keypress registered for each physical press. Missed or duplicated presses indicate a potential problem with the mod.

Punching holes out of other materials can work, too. You will only need a few square inches. I tried 50A silicone and 70 duro rubber, but both were much too soft for my liking. Vinyl or a harder rubber could be good. Gluing a dense paper material such as cardstock into layers would give you precise control over the thickness, though I imagine you would get little or no sound dampening effect with this material, and you may need to really jam it into the switch hole to get all the initial deformation out of the way, and the switch stem may damage the paper over time.

Side note: Before this mod, I tried using ball bearings. It felt okay, but it was loud, the balls often rattled during normal typing, and moving the keyboard between computers was a noisy affair. These flaws could probably be remedied by somehow sticking the ball to the bottom, eg deposit a really thick lube at the bottom using a syringe+needle, but I wanted something softer/quieter.

TL;DR: Instead of silicone balls, use a thin sheet of leather and a 2mm hole punch. Test it on one or a few switches before doing a bunch.