DIY Calculator-like Keyboard

[BillDee]

I don't know if this is a good place to ask about this,
but I want to make a customized calculator-like keyboard
for a retro-computing project.

I've looked around on the Net and did some searching,
but I can't find anything that would work.
I may not know what to search for or be using the
correct terms.

Most calculator keyboards use membrane-like keyboards
that are specialty made for that one application.
Including the housing and the "key caps" to make it all work.

I haven't been able to find someone who makes something
like that like you can do with cherry-like keys and caps.

I've thought about using cherry keys and caps with custom
printing, but that would make the keyboard pretty big.
Ideally I want to make the enclosure about the size of
a large, hand-held calculator. (including small display)
Or slightly bigger if needed, but not "huge" if at all possible.

I tried to find "single" membrane "key" units that I could
lay out in the design I want. But I don't think they make those.
They all seem to be in matrices with set sizing, most of which
were too big for what I wanted, or the number of buttons
was wrong, or the wrong layout, etc...

I thought about using actual button switches, because you can
get those in quite an array of sizes. But then there is the problem
of getting the captions on them, or "capping" them, etc...
Most of them would need 3- to 4-character mnemonic text words
on them, plus other symbols and such.

I thought about designing a "key cap" that could go over, and/or be
glued onto, a button switch, that could be printed using a 3-D
printer, which is not an easy job unto itself, but then I would
have to use some type of applique for the wording and such,
which rarely looks very good.

Unfortunately, cost is an issue, although these kinds of
things always seem to cost too much, especially since it will
most likely be a one-off project. Ideally I would like to be
able to do it fairly cheaply and show people how I did it so
others could re-create the project if they wanted to.

Any ideas, hints, tips, etc.?

[Findecanor]

The nicest vintage pocket calculators like 1970's Texas Instruments' do not have rubber domes but mechanical metal-to-metal clicky contact switches: like membranes, but with sheet metal instead of silver-paint on mylar. The key feel and sound is quite similar to microswitches, like what is used for mouse buttons. Keypads of microswitches on PCBs are readily available at component vendors and on eBay. You could also mount microswitches on a stripboard.
The chiclet keys are often loose in a frame just pressed against the switches. No stem, but maybe a nub on the underside. A keycap just needs a lip to stay within its frame.

Not all rubber domes are backed with mylar membranes. Some domes have conductive rubber at the bottom, which touch a PCB when pressed. On the PCB are lines close together so that the conductive rubber closes a circuit between them.
If you have a bunch of loose domes, a custom PCB and a frame for centring the domes on the PCB switch positions, you could make any layout.

Vintage desk calculators have often used (close to) the same switches as for computer keyboards and electronic typewriters. Sometimes they have been cheaper types because calculators was more of a mass-market item than computers back then. Some are linear (like Cherry MX red), and some are some combination of linear and rubber dome.
A few keycap vendors still make chiclet keycaps that can fit on Cherry MX or Alps - mostly for public terminals and such where keycaps can not be allowed to be pulled off.

As to making the frame and flat chiclet keys, I suggest you look into laser-cut and laser-engraved acrylic sheets. There are types of acrylic with a film on top that can be laser-engraved away to reveal a different colour underneath - similar to how many mass-manufactured keyboard keys are made.
Engraving and cutting acrylic could also be combined. I think I have seen a type of acrylic where the top layer looks like brushed aluminium, which I think would be perfect for a vintage pocket-calculator look.
There are several firms on the web where you upload a SVG and get the results mailed. You must of course adhere to the design rules properly to get an exact result. I have ordered once from Ponoko, but different firms have different options.
Makerspaces also often have laser-cutters.