At Nekomado's shogi booth at Essen, I just bought a solid (non-folding) wooden board with standard 9x9 on one side and mini 5x5 on the other.
I stand corrected - the board I bought in Essen has 5x6 rather than 5x5, which we found to our surprise when we went to play game on it yesterday. So we tried Mini Shogi with an extra row just to see how it works...
This 5x6 board is strange, since I don't know of any 5x6 Shogi variants!
http://en.wikipedia.org/wiki/Shogi_variant doesn't list any...
Does anyone know why there would be a 5x6 board on the back of a 9x9 Shogi board?