Great job!
Just a couple of things:
- The fuse on the 48V positive should be mounted close to the battery end of the wire rather than the reducer end.
- Your wire gauge is good (it could have been thinner on the 48V side but what you've got is more than sufficient).
- The yellow wire is correct.
- The accessory block looks terrific.
You do not need a relay because the trigger wire to the key switch does the same job.
As for your fuse sizes. The best way to work it out is to connect your multimeter across where the fuse would be (take the fuse out) and set to measure Amps. Switch on the accessory and measure the number of amps that it pulls. Put in a fuse that can handle just a bit more than you measured.
Cheers.