Which ever way it's done it's unlikely to be cheaper than buying a ready made one.(unless it's the $7,000 TZD 350?) Piece of cast iron would probably be best bet though, steel and stainless steel will probably glaze over too easy if your using hard compound linings., unlike disk brakes you may not be able to get a compound compatible with steel liner? I've thought about making a 4 leading shoe brake multiple times. Use a 'finned' automotive rear drum for liner and cast aluminium around it. (obviously after machining out 'rings' and thinning close to finished size) It should also be possible to fabricate a hub using this method if you alloy welding is good enough. The main issue is making brake plates, shoes shouldn't be so much of an issue.