Hey firehazard, I remember seeing your question about gunnite before and though someone with more experience would answer it, guess not. I know of the technique of concrete application you are talking about (also called shotcrete), I see it used all the time when they excavate large areas for tall buildings. If you question is "how can I make one" I'd say not to bother, it's complicated, messy, technical, and in the end will not be worth your time as you will probably have to throw it all away after one (possibly two) uses. your best bet is to go to a building supply store (take your pick: Lowe's, Home Depot, Home Hardware) and rent one. hope this helps!