There are programmable point-of-sale keyboards with matrix layout, black Cherry MX switches and n-key rollover that are only a little bit larger. They already come with flat keycaps, and keycaps that are twice as wide, spanning two key switches at once.
Big problem is that new keyboards and replacement keycaps are quite expensive, but used keyboards should not be a problem. Space bar is often also 2u large.
Brands: Tipro, Access IS, Jarltech. If you get a broken one that is too large, you could reuse it as a starting point for your project, cutting off excess keys and mending with wires, etc. or just reuse a part of the matrix plate.