Hi,
In germany we have build such layouts, and creating a norm. So we can play together and put one layout next to the other and we have a large TTTT-world. At TTTT meetings sometimes it take more than half an hour to drive from one end to the other of the whole layout.
Our rules:
-The base thickness should be 3cm. On every side must be a possibilty (3 cm above ground 4 cm wide) to drive on an other layout.
-The position of this pass thru is on your choice
-The layouts must be put on a normal table without demage the table surface (no scrws or nails under the layout).
-The size and the details of the layout are on everbodys own choice.
We have testet differnet kind of materials and way to build. Our favorite thing after testing is still styrofarm (=styropor or styrodur in germany)
The base is a 3cm thick plate. The lenght and the beam is on your choice. It is just a question of space and transport. A popular sice is 50x60cm.
The reasen for 3cm thickness is , so you have a definied "0"-high to drive from one layout on the next. The other reason is, you can buy the plate in 3 cm ;-)
Now you crush some pices of styrofarm and glue it on the layout. The best glue is wood-glue. Other one may destroy the styrofarm.
On you choice you can creat hills. With a cheap kitchen-knife you can cut valley or dry rivers.
To get a fine surface you glue tissue on the styrofarm. At the next, paint the whole layout in green colour (or yellow / brown for dust and sand).
In the wet colour you grit some strew (like "woodland scenics").
The strew you can buy in a modelrailroader shop. The modelrailroader use this too to create their layouts. It is availibe in green (gras), yellow (sand), brown or grey (stones),....
At the last you glue some stones, wood , sand or other details.
So you have a very light and easy to build layout.
If you want to have a parcours, so it is also very easy to plug some pinns or needles in the layout. That may be a problem if you use other materials than styrofarm.
At the picture you see our layout at the "Hamburger Modellbautage" (a model-fair in 2001).
best regards
Thorsten
www.mikromodell.de