Building a power supply for a Vic 20
Check!  
There's discussion whether the fuse should be placed before or after the transformer. I place it before. I'd use a 500mA, slow-blow ("T") version.  
I would use a metal case rather than plastic. Not that there would be a risk of the plastic melting &mdash; it doesn't get that hot &mdash; but metal transfers the heat better to the environment. I would use a ventilated enclosure if I could find it, though it will cost a bit more.
Mounting should be straightforward; if it's a classic EI transformer use a bolt and nut for all 4 mounting holes. Use spring washers so that the nuts don't come loose because of vibrations. A toroidal transformer often comes with mounting material, so use that.  
I don't think so. Success!