I use 1/4 inch tempered Masonite, attached with screws not glue. When it gets worn, turn it over for a fresh surface. When that side gets worn, replace it.
Line 24 caps are 9/16 inch diameter. Line 20 caps are 7/16 inch diameter.
http://www.tandyleatherfactory.com/en-usd/search/searchresults/1261-02.aspx
txbob