Before starting building, we wrote a simple code to test communication between an arduino and processing with 8 switches that presented 2 positions of the tray. In this point, we wanted to make sure that each switch could send its own value and processing could get it and trigger different images following the values it got from an arduino. It worked pretty well. So we moved to next step - making a prototype with a same condition with a completed project.
So we started making a prototype with this code and 4 wooden blocks and two arduinos instead of multiplexer. We made 16 switches on the board and connected wires and we talked about connections between blocks and a board. Initially we used coppers. It was easy to attach coppers on the wood with nails but to attach on the plastic, we should use glues and it might make uneven bottoms which was not neat. But coppers worked pretty good as a connector and that point Todd introduced copper tapes which were perfect for us. So we used it and finally these switches triggered images!