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.