Decent Top Box ?

I'm Birmingham based atm. In the process now of working out which fitting set I need. If I buy fitting set then plate then box in that order I dont end up with bits I can't fit I feel.

All the to boxes look huge but I teach and I need to keep stuff perfectly dry as I transport it home. I would normally go panniers but their cost is prohibitive and as I've mentioned before I filter, a lot.

I looked under the bike earlier in my lunch break and there seem to be some coverplates there where a bracket could go so feeling a bit more confident with that!

Again thanks folks, some brilliant advice and views.

Rob