I'd say check the whole black compartment behind the lens since you have crisp edges so it's not a rolling problem light leak. The standard places to tape up and seal are two small bumps at the top (which are probably hidden behind a proper mask in the newer holgas. Mine didn't come with a 6x6 mask) and all the little gaps/lines like around the holga name, across the front just below that bit, etc. It might even be coming from the shutter clicker/lever considering its position.