OK, I just looked at the mag and the plan. If you want to build it light weight, I would build a 1x2 frame, top it with1/4" Luan, then glue a one inch sheet of builders foam on the Luan. Use that as the base and lay the roadbed on that. With the foam, you can add to it to make the hills, and carve down into it for the stream etc. This is the way that I am going about building my HO layout but using 1x3's for the framing part.
This is not the only way to do it, just one way.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Sub-roadbeds can be made in different ways. Most require wood, although there are different alternatives. What kind of carpentry skills do you have?