You need a purposely-constructed wooden base or table(s), 60-70 cm high, that would exactly match the base map and be strong enough to support the weight of the model. Reinforce the base to avoid bending while the wet carton and papier-mâché dry up.

One side of the base table should measure less than 1.8 m, to permit easy access to otherwise hard-to-reach sections. It may sometimes be easier to work on two or more units rather than one, joining them on completion of the exercise.  Below are a sketch map and some images. Click to enlarge them.

Base Table Sketch design of a base table. The top is mobile to allow tipping it when taking high resolution images of the completed 3D model	Base Table details The top is mobile and the bottom reinforced
Base Table details The table top is made out of a 3/4 thick plywood reinforced by transversal lumbers
Base Table details The reinforcement of the base is clearly visible	Base Table details The completed model is tipped perpendicularly to the ground
Base Table details High resolution images are taken from a set distance