I use a simple design of pine framing with a 9mm ply top. For O14, the top is Sundeala to allow track spikes to push into the board easier, this is glued/pinned on top of the 9mm ply to guard against warping.
Hills are fashioned from 5mm ply spines then stuffed with crumpled up newspaper and covered in plaster bandage.
Trestles and wherever possible, adjustable kitchen worktop legs are used for support, these are affixed with a grub screw and are very quick to assemble. The trestles sit across board joins minimising their number.
Boards are aligned using wixroyd catches which allow some adjustment horizontally and vertically, plus coach bolts are inserted and tightened with wing nuts between the boards (2 per joint).