I've decided to beef up the scenic baseboard construction as I was not very happy with the flexibility of the 3mm MDF one. So I bought a 1200x600 sheet of 9mm exterior ply. This provided enough wood for 2 long sides, 2 ends and 2 stretchers bracings. Each 'girder' still measures 100m deep was built following the same method detailed for the MDF girder.

The build time was about 3 hours, which was a bit slow but it was after a full day of DIY on the house The result though is far better than the MDF one and I am really pleased with it; much stronger, more twist resistant but still very light.

Here are some photos of the built.

General view.

Close up of corner joint which was built the same way as explained by John Chivers and I think look very neat.

View showing how the stretcher girders can support track bed risers which I still intend to use.

Having now made the baseboard I could not resist in creating a bit of a mock with some cardboard, track and some building outlines. The following photos give some idea of how it might all look.

Overall I am quite pleased with it how it looks.