I built all my chicken coops out of scrap lumber so the only investment was for hardware cloth or else 1/2" x 1" wire mesh which is what is required around here to keep out the critters who fancy chicken for dinner. Then it is necessary to be sure to close the chickens in at night. The MO of your killer still sounds more like raccoons than anything else. I agree that cats are not a threat. If you have mink or weasels there they could also be suspects.
