Some notes for the 4th

So just yesterday I ordered some glass. It's supposed to be used for welding, but I decided to use it in the future for photography, as a dense filter. I was able to find a 50mm circular piece of #14, equivalent to about 18 stops of density. That's going to be SWEET! Now I'm going to be able to take long exposures during the day. I already have an ND filter that's 9 stops but I find that in some lighting situations, the filter is not dark enough. I was hoping that I could combine the two for even darker glass. I'm still wondering just how I'm going to attach that welder's glass to my camera, which has a 49mm filter adaptor (thread). any idea?