To make the board, I just cut the flaps off of a flattened corrugated cardboard box (I used a USPS flat rate board game shipping box), then cut a rectangle of flannel about two inches bigger on each side than the flattened box. I spread the flannel on the table (softer side down) and centered the box over it, then folded each side of the cloth over and hot-glued it about six inches at a time, so the glue wouldn't harden before I could press the fabric to it. I did opposite sides first, so I could pull it taut.
Here are photos of all the sets I made yesterday. I did it all freehand, but if you're not comfortable with the idea, I'm sure you could find simple patterns with minimal Google-ing.