What ever you do, DO NOT LINE WITH GRAVEL! Sand stone, flint, or finest marble if it's got any moisture in it (and chemically combined water that releases upon heating), then it WILL explode! I've dug my ground forge into pretty much every soil type now and I've seen a lot of things go pop! sometimes its only a bit of charcoal that gets moved, but I have retreated to a safe distance while the stoney shrapnel and embers died down!
Just use clay or clay tempered with sand, or just use soil if its sticky enough.
Asfor the straw ash as flux, that's a japanese thing. They use rice straw ash as the main component of their special recipe flux. I don't know how well it will work with steel but it seems to work OK for them with wrought iron. Same with sand (silver sand ideally). Borax is the usual flux these days as it actually cleans the surface of the metal as well as excluding the oxygen, which is all the ash/sand do.