I've managed to get someone to make these coasters up for me. To keep the costs down they're made from 2 sections of MDF glued together to give a 4mm recess.
The dimensions are 120mm x 120mm x 10mm. The recess is 100mm x 100mm.
I'll be making these up into kits of 4 and 6 with the classic mosaic designs, 8 - 12 mm tesserae, and also the Opus Sectile marble pieces.