We have had good luck using the galvanized lids from Monarch. Didn't see them listed on the website but some of the feeders have them on already. They do sell just the lid so call if u want it. They just sit ontop of the barrel.
http://www.deerfeeder.com/Products/QuantumFeeders/Standard55gal12V.aspx
We just put a rock or 2 on top of the lid and never had issues with it coming off. You can even use a bunge cord to secure the lid.
This place is not far from me and could swing by and pick up anything and deliver it next weekend.