I happened to construct my main keyboard to be pretty loud, without really trying. Here's how:
* If you press a key and bottom it out really hard, the bottom of the switch, switch pins or PCB should hit the bottom of the case. Ribs in the wrong places in the case bottom help with this.
* Make the plate sit really tight inside the case. That way, all vibrations in the plate are transferred directly to the case.
* Use no rubber feet so that the case clatters directly against the desk surface.
* Construct the case with unnecessary cavities that act as echo chambers.
* Use switches with very stiff springs. With users applying more force, that means more force in lateral movement (clatter) and a higher force (and louder noise) when the switch tops up. I used Cherry MX Clear, but a clicky switch with stiff springs would be worse. I think linear would be less noisy.
Other things I have learned:
* Use a too thin-diameter stabiliser wire that rattles a lot (K-Type. Input:Club had designed the keyboard but Massdrop had sourced the stabiliser. Input:Club's had rejected that stabiliser before launch but Massdrop used it anyway)
* Make top and bottom not sit really well together. Preferably, the keyboard should rattle if you shake it.
* Use keycaps in thin-walled OEM-profile in PBT plastic.
Higher keycaps lead to more wobble/lateral force and thus clatter inside the switch. Harder plastic dampens sound less. Thin-walled means less solidity and more space inside the keycaps for echo.
* Make sure that your switches are not lubricated. Either use switches that don't have any factory-lubrication, or disassemble, wash them and reassemble. (I have made a rubber dome keyboard feel very crappy by washing it...)
