There are two A3 sized base boards, built from foamboard. I used foamboard because it is very light and with proper construction strong and rigid. On the top of the boards I have added a layer of grey card over the foamboard to protect it from damage.
In this photo I was using the track templates to make sure everything still fitted in real life.
The two boards are attached together using nuts and bolts, to allow for easy joining and separation.
The holes at either side allow for easy access for wiring and use of uncoupling magnets.