+ 8 above numpad - can use 3 of the top 4 switches' LED traces for the 3 "lock" LEDs
+ 16 in an extra row underneath Esc through Pause/Break that are in cherry G80/G81 -3000 positions
[...]
Fits into cherry G80/G81 -3000 cases
There are features inside the G8x-3000 cases that would make those requirements a bit difficult.
There are both "newer" cases for keyboards with windows keys, and there are older
winkeyless cases.
These look almost the same from the outside, but they are somewhat different inside. The winkeyless cases are of higher quality with thicker plastic, so I think that they could be more desirable for this custom keyboard.
What complicates things is that there are poles here and there protruding from the top and/or bottom of the case through the PCB to hold it steady, and most of them are located in-between the function row and the numeric row where the extra row of function keys would go.
These poles are also in somewhat different positions inside newer and older cases, but the winkeyless cases have much fewer of them.
With a newer case, you would also have to cut away plastic for keys above the numpad.
Having an extra row of keys
above the function row old would also involve some cutting, but a lot less and you wouldn't have to be tidy. The plastic could be twisted off with a pair of pliers even.
Also:
* It is not possible to add a plate without a lot of cutting, and it is a bit difficult (and noisy and dusty if you use a dremel)
* Winkeyless and winkeyful keyboards have lock LEDs in different positions (but that is a minor issue)
(I know this because I transplanted a Chicony 5181 with plate-mounted Monterey switches into a newer G80-3000 case ... and then I got an older case and did it all over again. I should really post pics of that mod ...)