build one at a time. It's great to be enthusiastic but you will realistically only be able to make one very good machine or two poorer machines as you can't put the time into either.

I also wouldn't recommend a spinner unless you have been to a spinner competition (such as the one in a weeks time at Yeovil) and seen the insides of a spinning robot.

A cylinder is a tricky shape. Polycarb can be gently heated with a heatgun and with some brute force and ignorance bent into a cylinder. Aluminium or steel would more difficult depending on the thickness and grade of material. Any idea what material specifically you are going to use?