A replacement lid for the Thumler's Tumbler, 3 pound barrel is available (Part no. 905R-3). Have a spare lid handy in case of problems such as a large rock with sharp points causing the lid to tear or wear faster than normal.  To make your lid last a little longer, make a patch for the center of the lid from a piece of rubber, ( old bicycle tube, etc.) and attach it to the lid using rubber cement.  Be sure to check the lid on occasion to feel or see any deterioration, or thinning.