Reddit mentions of Isolate It!: Sorbothane Acoustic & Vibration Damping Film 50 Duro (0.10 x 6 x 12in)

Any kind of foam cut cut around the mounts/usb in the bottom of the case would help with that. I've heard sorbothane is really good but it's a bit pricey. Another popular solution that isn't foam is the shelf liner stuff usually used for kitchens.

It goes inside/in the bottom of the case. It's sandwiched in between the case and pcb. Link here- https://www.amazon.com/gp/product/B0084EXBFA/

I've been meaning to shoot a typing video so people can hear the difference. I have it installed in the exact same case you used for your build and it sounds 100X better.

I used this one https://www.amazon.com/gp/product/B0084EXBFA/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 I pasted all of it to the case, then used an exacto knife to make it fit well.

Huey from Top Clack has used this and did a typing video. It cut down the sound quite dramatically.

Shelf liner gets recommended a lot

Sorbothane is the good stuff though: https://www.amazon.com/Isolate-Sorbothane-Acoustic-Vibration-Damping/dp/B0084EXBFA/ref=sr_1_12?s=hi&ie=UTF8&qid=1481766863&sr=1-12&keywords=sorbothane+sheet

Questions about liner material and lube...

For liner material, what acoustic differences am I going to find between: neoprene, sorborthane, and rather generic looking "foam" rolls? They're all pretty much the same thickness, about 2-3 mm.

For lube, I'm looking at: Permatex dielectric grease, krytox 205 grade 2, trybosis 3204 and trybosis 3203. What would be good for stabs, what would be good for linears, and what would be good for tactiles? And if possible, is there anything that's a good "all-arounder"? Aka, something that could easily work for both stabs and switches.