Dime store mats may not be good quality.. i.e. acid free, but if you choose this route, an acid free scrapbooking marker could be used to line the white.
I would shop around. All you can really do. By the time you buy the marker and the mat, and drive around looking for it.. you might get a better deal on one already made, and will look more professional.
Try on-line shopping, too. |