I started with a large sheet of 0.5mm plasticard. This is hard to get but can be found from professional modelmaker's stockists, such as the 4D Modelshop in London, where I got mine.
The system is intended to be compatible with the Ziterdes modular gaming table that I use for all the games I play. This means that I had to cut the sheet to be 600mm square: when it's finished, I will mount the sheet on a piece of MDF or similar to raise it to the same height as the Ziterdes boards.
My plan is to have a simple grid system, with square blocks that can be moved around as required. Indivudual terrain pieces such as buildings will be mounted onto inserts that sit in holes cut into the sheet. This gives the impression of a raised pavement, and also hides the joins, making the whole thing look like it's a single piece.
After cutting the sheet to size, I cut four 200mm x 200mm squares out, leaving a 50mm border around the edge and 100mm gap between each hole (the roads are 100mm wide: not very realistic but they need to be big enough for titans and other super-heavies to maneuvre!). This means that if two boards are placed next to each other, there will always be a 100mm gap between each terrain piece.
Having cut out the four holes, I now have four 200mm x 200mm pieces of plasticard, which will form the base of each terrain piece:
More to follow!