Shroud for Model Air Boat
Brass will solder well - don't know the size of prop you want - may be a bit expensive. You can bend thin sheet round a former to make any diameter shroud.
Aluminium tube should do, if you can get an off-cut the size you want. Don't play around with aluminium soldering, which is a specialist job - but small pop rivets will work perfectly...
We don't know the size you are interested in, so it's hard to advise on materials. But if your boats are static models, card or thin plastic sheet will be fine. A slice from a plastic squash bottle may fit the bill?