if you want the cheapest, ask some FSU seller for a 42 snap-on plastic hood. I got one for a few bucks, round, made for the J-8 i think. (alex-photo was the seller). They might not have it listed but they might have some on stock. I asked, he had it.
If you want a bit better, or screw-in, go for a vented hood as said above.
The original Zeiss one is also nice but usually too expensive.
Edit: here's the vented one from "american eagle". A nice seller to trade with. (A propos: he gave me pos. feedback after the first email, BEFORE i made the payment!!! something unseen on e-bay)