Here we go with the brake drum forge project; thanks for the invitation, Stanley.
This is what I started out with. A folding portable riveter's/farrier's forge blower and stand, picked up at a deceased estate 'clearing sale' auction, some years ago. My files show that it cost me AU$21 [roughly £10]. These were originally fitted with a light sheet metal 'fire tray', which was relined with fireclay at each worksite. Most of those have rusted out over the years, and been discarded.
The 'Mk I' replacement fire tray, made from an old plough disc, to replicate the original reasonably closely. Wind shield is attached with solid riveted brackets, and has a removable 'gap piece' to accommodate long workpieces. I had to spot anneal the disc to drill the rivet holes; it's tough steel! This will accommodate quite a big fire, but needs a big fuel heap to get a deep fire.
The brake drum selected for the 'Mk II fire pot''. Desired features were overall size, a centre hole that's a reasonably good fit to the forge tuyere [or air discharge outlet], and the 'double lip' at the outer edge, to retain the wind shield. It was a bonus that the wheel stud holes were a perfect pitch circle match to the mounting lug bolt holes on the tuyere!
The 14ga sheet steel blank for the wind shield. The last of the piece which was also used for the Mk I's wind shield; this was an offcut from another 'recycle/repurpose' project.
The wind shield formed to shape. Cold bent over the anvil horn, using a rubber mallet to do the ends. As will be visible in later pics, I then trimmed one end back a bit, to make a wider gap. Again, this is to make heating of long workpieces more convenient.
For example, I've converted a fair few discarded 6' air leg rock drill steels to light crowbars. They make very good ones, as they're made from a very tough air-hardening alloy steel.
For a bottom mounted tuyere, a grate or air diffuser is a very good idea, on a number of counts. These are often made of steel plate or cast iron. But my thought was that light gauge stainless steel would work just as well, as it doesn't need a 'corrosion allowance' factor in its thickness. If it melts, I'll just make up something of thicker section.
I just happened to have a piece of ready-perforated 304 stainless steel sheet on hand, from a discarded clothes dryer drum end. This is backed up with a 3mm mild steel backing plate, again a leftover from a previous project [practicing flame cutting of circle segments, for a letterbox door]. Just needed cleaning up of the edges, and cutting/drilling the required holes. As well as clamping the diffuser plate, this blanks off the unused wheel stud holes in the brake drum.
Bottom view of the completed fire pot assembly, less mounting bolts.
Top view. The tabs welded on the mounting coach bolt heads are to prevent them from spinning, when undoing the nuts.
The earlier 'in action' pic in this thread was shot as available light; here's a flash-illuminated one of the Mk II fire pot's first heat.
And to finish off, here's the first completed job. A traditional tapered loop handle, on a small fire rake I'd made up for this forge. Entirely hammer and anvil work.
, as it can take several weeks to clean up heavily rusted items. But unlike acid baths, it won't attack good metal, if it's left in longer than necessary. I've seen mix ratios from 1+5 to 1+10 recommended, but being the thrifty type, reckoned I'd start at the more dilute end of the range. Which is working just fine. 
, I am sure you said 3 or 4 inch and we went for the larger and I think its a 6 inch chuck you have so that would make the blank probably the 4 inch size.