A missions board is just an object with the MissionsBoard flag on it. The only properties of interest on it is "board", which you should set to the master board (usually level2_missions_1), where minimissions are kept, and "nation", which defaults to british:mod.

NB. the way the master board works has changed, somebody who knows needs to update this

mission nation	board nation	Visible?
british	british	yes
british	british:mod	yes
british	french	no
british:mod	british	no
british:mod	british:mod	yes
british:mod	french	no
french	british	yes
french	british:mod	no
french	french	no

You can set a limit on the number of minimissions which a board can contain with the 'max' value. The default is only 4