I found three really good references for building model trenches.
This is perhaps a bit too elaborate for 15mm, but I might do something with it one day for my 1/72nd scale WW2 miniatures.
Inspiring, but more suitable for a diorama.
Way to go! Earth banks cut from a strip of timber framing, suitably painted and flocked should get us by. I might try and do some “dug-in” infantry as special bases, if I ever come across a supply of cheap heads etc.
Not exactly a trench, but a very nice model indeed is this from Thanos: http://miniaturesterrainpage.blogspot.com.au/2013/09/machine-gun-nest.html