Most of my freewheels are custom built using the Suntour Winner system, which allows building 5 or 6 speed freewheels for 120mm hubs and 6 or 7 speed freewheels for 126mm hubs. The system has different width spacers, permitting regular or narrow (ultra) cog spacing. I purchased a "shop board", which came with freewheel bodies, cogs and spacers that allow an almost unlimited variation in gearing setups. I recently even used this system to build a single speed freewheel for a restoration of a 1970s Mercier. The advantage of this method over just buying a single freewheel is that I could achieve a near perfect chainline to within +/- 0.5mm and could choose cogs from 15T to 21T.