Right on, not cockfighting
For plastic barrels I'd try making doors out of welded wire fencing, such as the 2"x4" hole size. Cut out a section that's a little bit wider than the opening, and cut one end off leaving protruding tines/pigtails that you can pass through the barrel and bend into "hinges" -- drill at least a hole at top/bottom, about 1/2" in from the edge, pass the wire through and bend it into a loop to secure it.
For a clasp on the other end, maybe install an anchor point in the barrel, such as an eye bolt or some other hardware, then simply use a carabiner between the fencing and the eye bolt for latching it shut.
Alternatively, if you have a roll of plumber's hanger tape and some bolts/nuts, you could make a small loop of the plumber's tape and bolt it to the barrel - that simple piece of hardware could be used for hinges or the clasping end.
Good luck