I have been working on this idea for a while: a compact portable 3D printer with folding print-bed and quick-detachable print-heads. The aim is to have a portable 3D printer that I can carry around, much like the Ultimaker 2 Go, but small enough to fit in a laptop backpack or a messenger bag. The design I currently have takes about 25x22.5x5.5cm when the bed and spool holder are folded and the print-head removed (the Ultimaker 2 Go is 28x28.7x25.5cm without Bowden tube and spool). The print-head and power supply can be stored inside the printer, only the filament spool will need additional space (for now!).
The print volume is 15x20x17cm, although this requires a slightly more complex mechanism for the y-axis, without it the print volume would be 15x15x17cm. The layer resolution is about 250 to 100 microns. I also have space to add a 12 cell battery, which would theoretically give about two to three hours of printing when using PLA.