I'm trying to replace a letter that disappeared from this church's sign. The sign is very old and no one had any information where it came from originally so I ordered a custom flat cut aluminum letter to match as well as possible. I'm trying to drill the stone and not having any luck. I've never drilled this surface. I was trying with my cordless hammer drill and a variety of bits(normal, titanium, glass, masonry). I can't get more than 3/8" deep. I'd like to get at least an inch. Any recommendations on what to try? The only thing I can think of is to go back with my corded hammer drill, just trying to avoid it since there is no power close by and my power inverter for my truck isn't hardwired. I have to hook it up at my battery. A picture of the stone is attached.