I wonder if not a custom-cut piece of plate foam would be enough protection for the PCB. At least against dust coming in from the top.

BTW, do not use silicone sealant. It emits acid when it cures. There are people who have tried using it for sound-proofing keyboards and had their PCB destroyed.

Hey, folks, my name is Honza, and I’m a mechanical engineering student from the Czech Republic. Ever since I started working with CAD software, I’ve been frustrated by the numpad being on the right. This is why I decided to design a fully featured southpaw keyboard, especially tailored to the people who do a lot of number crunching (Excel, CAD, or otherwise), or who even want to not have to choose between a numpad or more space for their mouse.    • Layout: Modified Southpaw 1800
    • Typing angle: 8 degrees
    • Front height: 19.4 mm
    • Mounting: Top
    • PCB options: Hotswap, solder – uses Unified Daughterboard S1 for USB connection
    • Materials: Aluminium case, brass internal weight
    • Estimated weight (unbuilt): 2.6 kg
    • Supported layouts: ISO, ANSI. Also supports stepped Caps Lock and split backspace.
    • Target price TBD based on ICThe inspiration was twofold – Upํr z Feratu (The Ferat Vampire in English) is a cult Czech classic movie about a racecar (said Ferat) running on the blood of the driver, which inspired the two-tone design of the keyboard case. In addition, the design of the top of the keyboard itself was inspired by the IBM Model M, and other massive early computer keyboards – I even added an IBM-esque badge!For  renders and visualizations, please visit my imgur gallery here.
I’m waiting for the prototype to finish manufacturing. In the meantime, I’m seeking feedback mostly about the final colorways available in the Group Buy, and the plate materials. Please, if you’re interested, share your thoughts at the link below:

GLWGB!!

Any update on this? Still very much interested!

The Kailh Box switches are the first to come to mind. The slider pushes onto a lever on the side of a box, and the mechanism is inside that box, separate from the compartment with the slider. That is supposed to make them more resistant to dust.
However, I dunno if anyone in the community has actually put that to the test.

The Kailh Box also has a shroud around the stem supposed to also make it harder for dust to get inside the first compartment.
Switches from other manufacturers with just a stem-shroud have also been marketed as "dust-proof" without being significantly different on the inside than a standard MX-type switch.

BTW. There are also some vintage boards with Cherry MX (or clone) that has like a fitted rubber or plastic sheet to protect against dust and liquids (colloquially called a "keyboard condom"). Those sheets would only fit on regular stems: not shrouded ones.
I've had one with a silicone sheet but it impaired key feel considerably, but I've seen police car keyboards with a thinner sheet.
I wonder what the best way would be to make one yourself. Perhaps 3D-print a mould, lay a sheet of plastic bag material and gently use a heat-gun? SIlicone would require a two-part mould, I'd think. Silicone could be thickened to be brushable, but the result would be stiffer and uneven.