Fantastic job on the pulley! Great idea.
If the diameter of the bare pulley was just slightly too small but you lay on enough epoxy to build it up, the excess will squeeze out when you wrap the belt around it for casting and take up the slack.
What did you use for mold release?
If you have motor heat issues, you could consider adding a small fan inside the box. I'm not sure what kind of air flow you'll get with just the wind. Hopefully a fan will be unnecessary.
It's going to look awesome with carbon fiber panels