Probably be good to ask this over in Shop Talk. A propane forge is fairly simple to build. I use a single burner for all of my forging; you can only forge a small section in each heat, and I don't do forge welding, so a single burner does all I need.
I recommend the burner kits from Zoeller Forge, and there are examples of forges on his website. A propane forge essentially needs a burner and a shell to give shape to the insulation and provide a support for mounting the burner. I've made them from paint cans and coffee cans before.