I built a second prototype. Here's a video showing the flexibility of the board and a short typing sound.
Mount structure
This is the part I was most concerned about, but it worked very well. This is a simple idea to achieve a top mount with a layered style keyboard. Use the nuts to secure the PCB or plate to the middle layer frame. An O-ring for the burger mount is optional, but with it, typing sound and touch feel is greatly improved.
The structure of this board is shown in order from the bottom layer.
Impression
Cheap looks, but the typing sound and feel are pretty good.
PCB flexcuts are as flexible as I imagined. The Alpha-numeric section moves up and down about 0.5mm to 1mm with normal typing. If press it strongly while it is stationary, it will drop about 3 mm. There is clearly less fatigue after long typing session compared to a hard setup like a brass plate.
I feel the sound was similar to Cherry G80 plateless board. I found that I like the combination of plateless build with plastic case.
For future builds, I'm considering designing a polycarbonate or aluminum case.
More considerations
* Mount position
The position of the mounting tabs can be improved. The switch near the tab feels a bit stiff at the bottom. I think it is better to move the tab to a place with a margin so that it does not overlap the switch position. After fixing this issue, I might release the first version on github.
* PCB durability
This PCB has a cutout and is 1.2mm thick. I don't know how much it will affect durability as a tradeoff with flexibility. I will evaluate it over a long period of time.
* PCB thickness
There is a tradeoff between flexibility and sound profile. I think that a typical thickness (1.6mm) will have a harder bottom out, but will give a deeper sound. The current build is the most flexible combination, but the combination of an O-ring for a burger mount and a 1.6mm standard PCB may be comfortable enough.