i had a great workshop at the transmediale berlin about building your own paper pcb. it was held by wolfgang spahn and we basically printed out a scheme which we then glued on a copper plate. from there it’s just soldering in all the parts… and we had build our very own custom made arduino UNO clone. full cost of this arduino is only 13€ and besides you can adjust it however you like, so it’s great for bigger stand-alone projects. with some practical experience it should not take you more than an hour to build your own arduino or other boards.
i highly recommend spahn’s paper pcb website, where you can download his designs and use them for your projects. for example there is a midi in/out board I would like to try out and other stuff.
a similar project is guilherme martin’s PAPERduino. here they put everything into card board, so they do not use a cooper plate. this results in more jumpers and slightly more messyness, however, still cool and open source as well.