Axle sizes

Hello fellow robot-builders!

 

For my next build it seems I migh have to do my own gearbox, I think I've worked out how to build it almost completely. The last issue I'm trying to figure out while I'm still in the planning stage is axles sizes and couplings. What is the best way to increase the diameter of an axle? Ideally, I shouldn't have to, but it all depends on what things I can get my hands on(I'm going for a one stage 30:1 reduction with a worm gear, since it doesn't take up much space and has builtin locking/braking).

 

My idea is to use tubes of the right outer and inner diameter, but I bet there's a better way. Share your thoughts!