Started work on the 2nd baseboard using spare wood from older layouts. The construction I am using is the typical 9mm ply top with softwood frames. However at one end I am leaving the frame open as need to get access to the top deck from underneath. I have seen a lot of people build a second deck and then have to thread wires down through two boards but for me that would be an absolute pain so leaving the bottom board out with a gap seems obvious. Underneath the bottom board there is a crosspiece so the frame forms a square. 

I prefer to use handtools and im using joints to lock all the frames together together. Screw and glue (and countersunk screwheads in so they are flush). Bit of sanding to do now and the top deck to add. Hopefully it will fit through the loft hatch....

I want to get this board sorted plus the top deck of this one and the previous baseboard so I can start working on the wiring which I will probably do the majority of (the layout control bus and the DCC circuit bus) before pining down the track while the board is up on one side so I don't have to spend quite so much time working upside down under the layout.