I have been using lightweight plywood baseboards for my portable 7mm ScaleSeven layout for around 17 years now. I evolved a very simple construction that meant that I didn't need to be a cabinet maker to get it right! I have just made some more boards for my foray into American H0.
Basically the materials are top quality 3mm plywood for the top with 6mm 'rubbish' plywood for the rest. I buy a full sheet, approx 8' x 4' in real measurements, from B & Q and get it sawn into the sizes I need on their large sheet saw. This ensures that all the pieces are square and, where necessary, the same width or length.
The sides, ends and braces inside are all about 4" deep. The internal structure is built up like an egg crate with notches cut - very roughly in my case - to half depth and the assembly just slots together. The 'compartments' are about 12" square and at least four of them also have diagonal pieces to add to the structural rigity and resist twisting. The ends fit inside the sides and they are hot glued to the top with the top on a flat surface. The 'egg crate' is then placed inside the upside down embryo baseboard, prodded into position and again hot glued to the top, sides and ends.
The internal structure pieces all have 2" dia lightening holes cut at about 3" or 4" centres to reduce weight but also - very important this - to allow the wiring to pass along the boards.
Next, every joint is reinforced with glass fibre tape and epoxy resin and this imparts the majority of the strength to the structure. Varnish or paint when finished. 'Ups and downs' can be accommodated by cutting out the top sheet, notching out the braces to accept it and then depressing or lifting the cut-out and hot glueing followed by the glass fibre tape. (I think that is probably what the Yanks call 'cookie-cutter'.)
This construction is very light, cheap and simple to build. No accurate corner blocks are needed and woodworking skills needed are earlier than 'beginner'! My ScaleSeven boards are each 4' x 2' and weigh just 7.5lbs (3kg) each. They are very strong and in 17 years haven't twisted or warped.
Obviously the deeper the side members the stiffer the final structure. However, I reckon that 4" is about the optimum. It isn't worth going any deeper.
Paint or varnish the finished baseboard. A cheap paint is adequate. Colour and glossiness to taste.
2008-03-15: Updated to HTML 4.01 Strict.
2009-01-01: Typeface updated.