I just use a red hot pen knife to cut out a small triangle from the plastic square hood. I did that the first night I bought it. I was a Leica virgin then, didn't know how much it cost. But I don't intend to sell the Leica stuffs I bought new anyway.
I think Ebay did have some third party metal vented hoods. I vaguely remember it was 5-star brand or something like that.