I'm always looking for cheap wheels of various kinds, and rarely find anything for under £1 a piece. 

How can I make a nicely round disc out of metal (aluminium) or wood with a central hole, if I don't have a lathe? 

The best idea I've had is to use a scroll saw or similar to cut out a near-circle from a piece of plywood, then drill a hole in the near-centre with a pillar drill arrangement, and some how attach it to a power drill, using a block of sand paper or similar to make the outside more circle-like, around the axle hole.

Can anyone recommend any alternatives? E.g. for 4cm diameter wheel, 10mm thickness, 5mm axle hole?