Here are two ways of doing it using home made dug-in markers.
I don't really like that approach as it takes up a lot of table space.
The picture above shows an alternative way of depicting a trench using the same markers, but of course now there is no room for the troops.
Investigations will have to continue.
Finally, in searching for the dug-in markers I came across these wire entanglements that I made for Crossfire and while a tad warped (they were just made on cardboard, probably from a pizza box) they can possibly still do service.