Had a new Thermatrue door installed, but they left a 3/8" gap between the new metal threshold and the old pitched sill. Is there any thing I might "inject" in that gap to make it solid enough to step on and not flex? They wanted to squeeze caulk in the gap but I didn't like that idea. I figure there must be some sort of gunable substance in there to accomplish the effect of a flat base that they were supposed to use. Any suggestion or idea gladly welcomed.