There is a Mayfly Protoshield rev1 – but I can’t find if its for sale anywhere.
I have one, and no plans for it right now so if you wanted it I could send it to you.
The details are here, but no gerbers. With gerbers it would be possible to order the base boards directly from oshpark.com
I have a base board defined in my PCB package, I could look at what it would take to generate a PCB and then you could order it directly.
This would be the simplest, small spacing 2.54mm/0.1″ using,
but practically speaking, it could be any connectors that you define how they fit round the Mayfly 2×10 connector.
I have a PCB that fits on the analog side – for Electrical Conductivity Stream Disconnect for (which I’m hoping to to write up pretty soon ) so it would look something like this..