I was also looking for a small and lightweight mechanical travel keyboard and just decided to go with the prebuilt Planck EZ from ZSA. The reason being the case. I wanted a case for some protection because it's a travel board but it had to be light. The EZ's case has a top side that doubles as the plate, so it doesn't add that much weight. My EZ weighs 297g including switches and keycaps and you can barely go smaller than that.
