If you can’t do an actual recessed piece I would do something just like is shown at the bottom of Etranger’s post: just the sides, in multiple lengths. That way you can put them wherever and as wide as you want them to be. Dust the bottom area with some model railroad ballast and there you go! I use the model ballast quite a bit, especially for trails through jungle. A very small dust-buster vacuum really helps with clean up!
"I saw this in a cartoon once, but I'm pretty sure I can do it..."