Black landscape mat works fine and is inexpensive. Not my choice for a finished layout but I used it on a few temp jobs and it looks great.
Inexpensive and I just stapled gunned it to the under side of the fascia.
Cabinet style fronts on magnetic latches are great too. Only did one of these but it was really nice for a solid look that was quick to remove.
What ever you do use a solid dark color. This brings the eyes to the trains instead of the skirting.
I did use landscape fabric for my skirting. Can't beat the cost, especially with a couple hundred feet needing to be done. There is one minor drawback with it, and that is it's a bit stiff, and doesn't like to hang in a "re;axed" way. It seems like every time you touch it, you have to go back and adjust it.
Because the fascia on my layout has a lot of curves, I created my own rail to hang the skirting from, using 3/4" plywood strips for the straight parts, and laminations of 1/4" Luan plywood for the curves. Here we are gluing up a curved rail.
This section is ready for fabric.
We just stapled the fabric directly to the rail, adding pleats along the way.