Don't rule out wood too quickly, but first you have to realize whatever you do (wood, foamboard, etc.) will cost you some time, money, and space, both for storage and when it's set up. 1/3 or 1/4 scale is not small or convenient like 1/6 scale.
I don't know if you have any "do it yourself" home stores like we do in the US (Home Depot, Lowe's, Menard's, etc.), but you can buy a full sheet of plywood and these stores will cut it for you for a nominal charge. For 1/3 or 1/4 scale, a rear wall of about .8 meters to a meter high and 1.8 to 2 meters wide works well. I gives you a comfortably amount of space to work in without the top and side edges of the walls showing in all your pictures. If you keep the walls flat (or very low relief) you can store them behind a door or under a bed when not in use. I don't know what you have over there, but my plywood of choice is 1/4" (5mm) luan plywood. It is fairly light weight, rigid, has one very smooth side, suitable for painting or applying wallpaper, and it is much more durable and water resistant than foamcore. If you don't have a table big enough to house your room, you can invest in a piece of thicker plywood, about 2cm thick and temporarily set up your room on a bed. Adding a strip of wood to the bottom back side of each wall would allow you to attach the walls to the floor with screws, bolts and nuts, or spring clips so it can be assembled and disassembled easily. Add these strips of wood to the side edges of the two walls meet so they can be clipped together as well. These wood strips will also help keep the plywood flat and add support to the structure, which is desirable for attaching clip lights to the top of the walls. Oh that's right, you're probably going to want to light this room too. That's a whole other issue.
If the wood route seems a bit daunting, you can go the foamboard route.
Fome-cor (a.k.a. foam core) is the copyrighted trade name for a paper-faced foam board comprised of extruded polystyrene foam with clay-coated paper facers.
http://graphicdisplayusa.com/en/product ... cor-board/There are many similar competing brands and foam core (like kleenex or xerox) has become a generic name for these foam board products.
Fome-cor is light weight, fairly rigid, and easy to cut with a utility knife. The downside is that the thinner more affordable varieties do not stay very flat without framing and will still warp even when framed. Painting with water based paints can cause even more warping. You can frequently find pre-cut pieces at hobby and art supply stores. If you are unfamiliar with this product it would be a good idea to trek down to an art supply store that has some and check it out.
If you are in this for the long haul you might want to think about some of foam board's bigger brothers such as Rynoboard and Gatorfoam.
http://www.foamboardsource.com/medium-- ... board.htmlhttp://graphicdisplayusa.com/en/product ... gatorfoam/Both are similar to Fome-cor but use different materials in their construction. Rynoboard is a heavy-duty exterior sheet laminated on each side of a high density, expanded polystyrene core (bead foam). Gatorfoam has the same extruded polystyrene foam interior as Fome-cor but uses a exterior veneer of wood-fiber making it extremely rigid. The surface is similar in texture to smooth kraftpaper. Ryno and Gator are much more durable and rigid, especially the thicker versions (1.5cm+) and these thicker boards are very flat and more resistant to warping when painted with water based paints. Of course there is a price to pay for these features and both Ryno and Gator are more expensive than the similarly sized foamboards (Fome-cor). In the long run the features are worth the additional cost. You may not find these at an art supply store. You might try a picture framer. I've had posters dry mounted on both Ryno and Gator at a local picture framer.
Another alternative would be to build wooden frames, stretch muslin or canvas on them, and then paint them as desired (kind of sounds like theatrical flats). You could use the pre-cut stretcher frames and fabrics (muslin or canvas) available at art supply stores. The advantages would be their light weight, they wouldn't warp much and can be twisted easily back to flatness, and they are made to be painted. The down side would be the fabric is easier to poke a hole in than foam boards, but they can be repaired or the fabric can be replaced.