Tim,
You could get a lot of opinions about this. Most of them are likely to not work with your available materials. I recommend you do a search on the web and find something close to what your materials and situation will allow and go from there. Your point, of course, is to not be lead down dead end trails to get a usable forge.
You have to ask yourself "will I be making more knives or is this the only one?" That will influence which direction you take.