I don't know what you have in terms of tools, but if you have any snap ring pliers, one of the larger diameter tip sizes should fit and work well without marring anything. Also, if you are careful the tips of a smaller pair of needle nose pliers (or round nose pliers) might fit into the holes, then just spin them to unscrew the cover.
You can also possibly make something satisfactory out of a piece of scrap metal. Just find something long enough for you to hold onto and get some leverage, drill a hole in it for the ratchet's square drive to fit through, and then drill and tap two 6-32 or 8-32 screws (or possibly even some Tek type self tapping sheetmetal screws) through the piece of metal on either side of the "square drive hole". Thread the two screws through and a quarter inch or so out the other side so they become the pins needed to engage the pin holes on the ratchet.