There are several choices for a wood effect finish.
I made a video showing the construction of my bed /seat base for my Transit Mk5 campervan which might be helpful
http://youtu.be/w48QqVXiTEw1. Use 12mm sheet Ply,Contiboard,mdf,etc for the frame and finish. Screw and glue the frame together and use plastic jointing blocks. Use self finished 12mm Furniture board with a wood effect finish ( Wickes )only where it shows like on the front face . This will be an extremely heavy base !
Furniture board comes in various thicknesses but 12mm would be the optimum thickness if you are using the board as the structural frame.
2. Build a lightweight softwood frame 32mm X 18mm (or 25mm x 18mm) and cover it with 3mm Plywood sheet ,using 3mm wood effect finish Furniture board only where it shows .The framework need only be 18mm x 32mm maximum provided the frame is glued and screwed and the 3mm furniture board is glued and screwed to it to make an extremely strong box frame.
I made my frame out of 25mm x 18mm covered it with 3mm Ply and faced the box with 12mm furniture board and it takes my 18 stone with ease . I had to use 12mm furniture board as I couldn't find 3mm in the size I needed.
The top is a hinged lid made of 9mm Plywood so you can stow stuff inside .
3. A third option is to use vynal wood effect which you buy on a roll to cover all your cheap sheet wood again where it shows. This is how the majority of caravans and coach built campervans are made . Once the vynal is fitted you cannot tell it isn't real wood .
http://www.amazon.co.uk/Effect-Self-Adh ... B005K7ZKDAWood effect melamine faced furniture board supplier who is happy to send free samples they supply in all thicknessess at 8' x4'' sheets and will precision cut to your measurements
http://www.bridec.co.uk/swatches_wood.php