Steve Smith G0TDJ of ProjectAVR writes:
The minDUINO is designed to be easy to assemble, so it is a two layer through-hole design, not SMD. It has headers for FTDI, ISCP and port breakouts. Another project I designed was VAYU-NTX, a High Altitude Balloon Tracker. As expansion for this, I designed two small daughter boards for experimenting with. I followed a similar idea with the minDUINO and placed the breakout headers together. Later on, I may design a bespoke board but its just as easy to use stripboard/perfboard paired with the appropriate header socket.
Design files are available here.