If you decide to go with plaster-based scenery, you might want to try a product called Durabond 90 (there is also a Durabond 60-the number refers to the setting time in minutes-there may be other setting times available.) I have used it directly over window screen without bothering with the paper towels. I prefer aluminum screen as it holds its shape better than fibreglass. Either way, support it well. This stuff will also stick to wood, plywood, or dry plaster-it helps to wet these surfaces slightly beforehand. For use directly over screen, I mix it to the consistency of a fairly stiff cake icing and apply it with a putty knife or scraper of the appropriate width. If you want a reasonably smooth surface , after you have applied the plaster, use a cheap synthetic brush to smooth the contours, dipping it often in water. Depending on where you wish to use it and the effect you want to achieve, you can mix it to almost any consistency and it will still set up in the allotted time. After it has set, it is possible to scrape down high points,etc. Once it has dried completely (up to a couple of days for a thick application), it is extremely hard, although it can still be drilled. An auto body file is also useful for shaping the hardened plaster. I paint the finished landforms with thinned latex paint - unthinned paint or water-based stains also work well. This material is also great for making bridge piers and abutments, etc. I make moulds from .060 styrene and wipe on a thin coat of vegetable oil with a paper towel to act as a mould release. Durabond 90 is a US Gypsum product (CGC in Canada). I purchased a 15 kg (33 lb) bag at Home Depot for about $18.00 Canadian. In spite of all of the above, don't restrict youself to only one method - in some situations, foam might be a more suitable medium. I hope this information will be of some assistance.