I took some pictures of my initial prototype of my silicon minimalist case for my 40%. The silicon I am using has an adhesive on one side, so this is four layers of silicon all glued together. Basically, the PCB and solder joints sit on the bottom plane with the teensy suspended in the cutout. The top two layers are cut to be the same size as the PCB, so you have to put the case on one corner at a time and it fits snuggly around the PCB.
My initial plan was to attach the original bottom plate to the top plate through the silicon, but when I started playing with it just like this I kind of liked it. The PCB is stuck in the top two layers, so its not going anywhere. Also, the bottom surface is an entire plane of silicone, so it sticks to the desk super well. Because this keyboard is so light to start with, it does tend to move around if you don't have good bumpers, so I actually like it like this.
Anyway, enough talk, more show...
PS - Cutting silicone is a PITA, so if I did this again, I would build up all the layers with the cutouts before I cut around the outside. Then I would cut all 4 layers around the outside at one time. This would make the outside edges look nice and clean. Trying to 'touch it up' after the initial cut will just make it look horrible (as you can see). The first cuts are all nice and clean, I just laminated them together after cutting them and it was tricky to do without a jig or anything. Lessons learned...