I got inspired by Gale Force 9's Sci-Fi turn counter, which is very nice, and built my own out of plasticard. I used 2mm thick plasticard, and two 3 x 2 mm magnets. This PDF has the templates and quick instructions - in essence glue the templates to the plasticard using weak glue (I used a stick of paper glue), cut to size, remove templates, then stick the faces to the plasticard with stronger glue (I used PVA glue). I also laminated the faces before glueing them.
Here is the result: