I think that this generator principle also works for a cast iron housing but an aluminium housing is most standard for small asynchronous motors in Europe. The disadvantage of a cast iron housing is that more of the magnetic flux will flow through the housing but as the distance in between the housing and the magnets of the armature is rather large, the losses in the housing due to eddy currents will probably be still low enough.
The material for the stator body can be any material if it is non metal, so it should guide no magnetic flux nor an electric current. In The Netherlands, Delrin is very generally available. It is one of the brands of POM (polyoxymethylene). Another brand is Ertalon. The main advantage of POM is that it keeps its measures.
The advantage of an 8-pole armature is that the stator winding is rather simple. For bigger housings you can use larger magnets or you can increase the pole number but the pole number must be devidable by four for a 2-layers winding. So the next pole number is 12 en then the stator will have 18 coils.