At the recent - and excellent - Daventry games day, I mentioned a company that did Bellona-type trench and ruined building scenery. For those interested, the link is below. It is 1/72 scale rather than 15mm - but there again, trench systems are often a bit mean in size, especially for the bigger affairs of the late war. It looks like two of the trench layouts would cover 4 ft x 1 ft and so include the rough terrain in front. I saw them at a show and they looked good.
http://www.amera.co.uk/product.php?range=a
Regards
Simon
Ironclad do a nice trench system for 15mm