These pochade boxes are useful, but it seems very difficult to get hold of 8" x 6" boards - why is that?
There are a couple of suppliers which supply this size board.
DRDB0806 Daler-Rowney Daler Board for Oil Painting 8in x 6in (Each):
PWCB08064 Frisk Canvas Board 203 x 152mm (8" x 6") Pack of 4
Another inexpensive alternative is to get MDF commercially cut to size for the box.