The Root Maker Company makes a bottom base called a Root Trapper disk. It is a felt type material adhered onto a flexible black vinyl material. Root Trapper disks, themselves prune the bottom roots by trapping the root tips in the felt material. See link below. You can also use polyethylene sheeting, cut to whatever size you need, it works OK but, of course, you then lose the bottom root pruning advantage. - Millet (1,249-)
http://www.rootmaker.com/products_bases.php#jump