Something I'm considering making (because I've realised my largest table is a tad smaller across it's width than 3x 25mm, and quite a bit smaller along it's length than 6x 25mm) is a felt-lined modular playing board, made up of a number of square boards, of two different sizes, and a number of slightly oblong boards.
Most of the boards (the oblong ones) will have a lip along one side, for edge boards, four (slightly larger sized square ones) will have a lip along two sides, for corner boards, and a few (25mm x 25mm square boards) will have no lip, for centre boards.
These would then have rare-earth magnets embedded in the sides to hold them together when in use, and could be configured to hold any size playing board (I might even make one board with a lip round all four sides, for a single-tile-size board, just for completenesses sake!)
Not sure of what material to use, yet - possibly MDF - but that would be quite bulky - or possibly thick card - though that would be a tad flimsy.
EDIT:
Ooh... Actually, I've just realised, I could do it with eighteen 25mm x 25mm boards, all with magnets embedded in all four sides, and then eighteen 25mm long strips, with a width equal to the thickness of the boards plus the thickness of the game tiles, also with magnets in them, which would then go around the outsides of the playing board...