This is awesome stuff, thanks guys! Much appreciated.
I've been letting my clay cat litter and wood ash mixture dry slowly, and it seems to be slightly more stable now. Still cracks in it, but I don't think it matters too much, as Sleek said. I'll just plug it with more stuff if I need to.
The purpose of this forge is to make my own arrowheads, and I want everything to be as traditional as possible - I actually made the bellows long before I had any plans for them, as I thought they'd be a nice project. Depending on practicality, I might end up using electronic blowers / dryers etc if I need to forge in a hurry.