This furnace will be able to handle melting aluminum and bronze with no trouble. In theory it should be able to melt iron too but that's probably pushing it.
I've been using my furnace for a couple years now and it has worked out quite well. My design is based on information I picked up from the backyard metalcasting forum. It's a great resource and community. There are a bunch of furnace builds there exploring a variety of ideas. I recommend checking it out. I'd also read this comprehensive guide for a general overview of metal casting and furnace design.
Step 1: General Plan
The furnace is really simple. It's just a metal container with a lid, a drain and a hole for the burner. It is filled with high temperature insulation to retain the heat and a sturdy inner lining (hot face) to withstand the flame from the burner.
I based my design around a small metal barrel that I got from a salvage place. The sheet metal was thick enough to weld to (pretty much) and not galvanized. The zinc coating on galvanazed metal containers will burn and make a noxious smoke so don't use it. Otherwise any metal container should make a serviceable shell for the furnace. I used bits of pipe and angle iron from my scrap pile for the rest of the metal parts.
I made my own insulation because it was substantially cheaper than buying commercial insulation and I just wanted to try it. I did buy a bag of high temperature refractory cement for the hot face.
The shell is filled with insulation leaving an airspace of roughly 2" on all sides between the crucible and the inside of the furnace.