A map board is used to map geology out in the field. It is simply two pieces of plexiglass or perspex duct taped together on the long edge. For Spring Field, the syllabus recommended 11x17" plexiglass. But you may find it useful to also have one that is closer to letter size. 11x14x0.8" works pretty good and may fit in your backpack. Don't get plexiglass that is too thick or too thin else they will break easily or your map board will be too heavy.
Ace Hardware in town can cut plexiglass to order. The Home Depot in Woodland has precut plexiglass which may fit your needs. You may want to file down the corners and edges of the plexiglass. This will prevent the plexiglass from cutting into your skin and cutting holes in your pack.
You will need to tape basemaps within the interior of your map board using low tack tape such as painter's tape. This tape won't leave much of a film on the plexiglass and is sometimes easy to remove from your basemap.