The baseboard for the main yard is 1200mm long by 450mm wide. The fiddle yard(s) will be 1200mm x 150mm
Additional modules will either be 1200mm long or multiples of, for example 2400mm long.
The baseboard for the main yard 9mm plywood which was some slightly warped sheet I rescued from a skip on a local driveway. It was an off cut so to get the length I need, I had to split it down and make two pieces.
Supporting the top sheet is 44mm x 18mm whitewood batterns from our local B&Q. The batterns were reasonable straight and have pulled most of the warping of the plywood out and it sits on my workbench without too much "rock".
The baseboard for the second module (Shelly's Sidings) is constructed from 9mm plywood for the main surface, the "battens" and the end plates.
The "battens" are 44mm so the top of the baseboard will match the top of "Dib's Yard" perfectly.
Baseboard alignment will be taken card of by 8mm metal alignment dowels. A jig has been made to ensure the dowel positions will be consistent.