I use cheap, stiff, wood pulp mat board (the best and only use for this type of board, IMO), white on one side (toward enlarger), black on other (toward print), of all sizes, some with cut holes of various sizes and type to fit each job. Some have holes taped with painters or gaffers tape with which to fine tune their "design". BTW, I use the same board, taped pieces to light wire, to construct dodging wands, as needed. In either case, proper technique requires that you hold the cards far enough away from the print that its penumbra will not result in a hard edge burning line, and keep them moving. Even I find it somewhat stunning how low–tech my required "tools" are for burning and dodging fine art prints.