To get OP out of hand filing all those corners I started researching the current year price of a 14mm sq punch (greenlee style) and then gave up and tried 9/16in instead, prices have gone up a bit since my dad was making vacuum tube projects with greenlee punches...

Its probably cheaper to watercut than to buy and use a punch (and hope for perfect alignment)

The problem with lasers is heat distortion that is a huge amount of cutting and the odds of it not turning into a pretzel are not so good.

A weldment with CNC carved rows and columns welded together hopefully into a square array is not ridiculous. In fact its probably the cheapest option. And you could use a nice aluminum and then anodize it some crazy cool color. If not aluminum and tig welding, maybe a somewhat less ambitious steel and silver "solder" (brazing, really) would be more realistic.

i was thinking 'water jet' too, but i think your welding process is potentially just as nice with a lower bar of entry (assuming you have a tig sitting next to you). you'd have to find a really friendly water jet shop to set up such a small run. if you had a mill, even a little one, you could take aluminum flat and cut channels so that the flats interlock. at that point you could just put down some tacks or even use fasteners.

if we want to lower it further, i think your brazing suggestion is good to. brass is easier to braze with a small hand held propane torch than steel i think. also more local hardware stores carry products like stay-brite and appropriate fluxes for copper alloys.

is there way to use half thickness so the top tab grabs on two edges and the bottom guys on two others?

It's possible to get waterjet cutting in low quantities with small setup charges. We have a good number of people ordering custom keyboard plates and cases from us at Big Blue Saw.