Short answer is Yes, you can use a dented drum. Do take a mallet and dent it outwards.
It is recomended, to use a drum with a removable lid. But not everybody can find one. Use what you have. One of the beauty's of cob is how easy it is to rebuild / make changes... just crumble your cob up and rehydrate! So later if a barrel with removable lid comes your way it will be easy to install.
Not all who wander are lost... J.R.R. Tolkien
You didn't tell me he was so big. Unlike this tiny ad: