oTEMPE5To has posted above his experience with taping the top of the PCB. Initially, these are his findings for taping the bottom: The tape mod - YouTube
I’ve done this mod to two of my boards now and I’m really happy with the outcome. Especially since it’s a mod that’s cheap, easy to do, and easy to undo
Tape on top should work if you do it before you insert and solder the switches.
It’s probably safer too since there are no components on the top.
In the same vein of this thread, @keebs will be offering a sound altering solution in the near future. He has a survey to help the process along.
For most builds I have some sort of case foam. I’m considering taping the inside edges of the case of some cheaper plastic case builds, in addition to the case foam, to see if that makes an additional difference in sound profile.
actually… there’s two more coming. I’m preparing right now the second post on top of the thockpads from yesterday 
I want to offer a “cheap” way of having all kinds of solutions at hand. from around the switches, inside the case, to the mat.
btw. here’s a tip I want to try out and I’m very confident that it works: get “nano suction tape” it’s transparent, mostly double sided, very thick (1-2mm) acrylic tape. it sticks strong, can be pulled off easy, leaves no marks, and is heavy.
What’s next? Practically anything can change acoustics, not necessarily for the better but it’s a wide-open area to play in. How would adding a few coats of nail polish inside outside the switch change its sound? Quite a large variety of the stuff out there also. 
BTW, I tried mods like taping switch bottom while back but the result wasn’t memorable. The difference is that, when I tested mods before, I tested on only a handful of switches, not a full board in use like people are doing now.
So the lesson here is: don’t judge a mod until you tried it a full board in use.
Tape mod tested and verified by Taeha:
2 hours 12 minutes in.
“verified” that it does “something” 
the sound difference is quite contrasting. I’ve done it myself and I think it reduces some hollowness.
I just did this to my Space65 CyberVoyager (painter’s tape + gaffer’s tape) which even with a small void under the PCB and with case foam is a bit hollow sounding, and it did in fact get rid of the echo. Pretty happy with this mod so far.
Prior to trying the 1mm PE foam on Polaris half-plate, I tested some switches to see which would produce the best sound: retooled black, Cream, or Gateron Black Ink. As you can see, bottom part of the switches don’t sit fully flushed against the PCB.
Of the three, I liked Gateron Black Inks the most. Not much clack but loud low-pitched thock w/a touch of wet popping sound. So I think there will be plenty of which switch in a particular case and plate will sound the best in the Tape Mod discussions.
UPDATE: I won’t know what soldering may have done to the PCB until I desolder them. Hopefully, no damage except toxic fume. 
UPDATE2: Happy to report no damage to PCB and switch from soldering with PE foam between PCB and switch.
Polaris half-plate with tape mod as well as PE foam on PCB mods.
Enjoying the sound so far: low-pitched knocking-on-plastic sound. Uneven switch heights is just visually annoying. I’ll fix them later if I don’t get tired of the sound.
I also taped up full brass plate build and there was only minor difference from the above build which suggests tape mod effect significantly outweighs PE foam mod. Given that later mod also comes with other issues, particularly with half-plate and PCB-mount builds, I’m going to remove the PE foam later.
Taped two more boards:
Aluminum Tofu + hotswap PCB + POM plate + retooled blacks = Did not improve nor lower pitch. Instead sounded flat. No good.
Acrylic Tofu + DZ60 + full brass plate + Hyperglides = Sound improved, tighter and more rounded. Keeper.
thanks, good suggestion. I ordered the nano suction tape from aliexpress (pretty cheap):
I hope that one has film on the other side - usually the double sided has red film on one side
so the description says double-sided tape. On the other hand, why does it have to be double-sided at all? It only sticks to the PCB on one side.
one sided is usually <1mm and double sided is up to 2mm.
Tape modded my otherwise too sharp sounding QXP and the results were quite satisfying. The QXP is now back in the regular rotation.
ouh is 2mm better than 1mm? there were also offers for 2mm tape, but I thought 2mm can be a bit thick, for example when case foam is installed.
