I chose to use 5mm Foam-core board (expanded polystyrene sandwiched between thin layers of card), I found A1 sheets in 3mm and 5mm thickness in my local WH Smiths. I last used this for Pen-Y-Bryn Quarry, and it is dead easy to work with - simply cut with a knife (the Stanley type works well) and stick with a hot glue-gun. The tricky bit is working out the pieces needed for the 3-D jigsaw, but once stuck together "egg-box" style it is very strong, the pictures below show the construction method.
The fascias of the board will be clad in thin Ply (the 4mm stuff, here being used as a cutting mat!), which will also be used to add a back-scene along the back edge. The ply neatens up the appearance but more importantly, protects the foam-core which is strong but vulnerable to dents and tears.
This form of construction makes different shapes and levels easy to accommodate. The footpath was partially cut out, then after the board was assembled it was removed completely and dropped.
At this stage the layout is easily picked up with a little finger, and is more of a sail than a model railway...