Can anybody advise me?
What kind of advice are you searching for?
I think I just need two metal bars that will guide a flat strap, and these metal bars should be lower than the rubber feet.
I guess that should work. You will probably need to attach some paddings for stability though to prevent the halves from rocking on your thighs.
I am using this setup:
It is rock solid. Works fine when standing. When lying down, the belt has to be loosened, as it is designed for elbows to be slightly behind the body (for proper posture), so the floor is in the way of that at default height. Also works great as a tenting setup when the keyboard is sitting on the table.
Screwed together from off-shelf parts, no metal cutting/drilling needed.
As for connection, I am using a 5cm usb extension cord and a magnetic adapter, so it is easy to quickly connect/disconnect.
Another idea is to use two sheet of leather, roughly triangular. Screw the UHK to it using washers and M4 screws, and make holes for the belt to go through.
Again would probably require padding for stability.
P.S. discourse prevents me from attaching more than one photo. This is annoying!