I just finished a plastics project in my venturi.
Weekend Venturi Project
The best advice I can give you about cutting it is to find a good water-jet company. There is probably one nearby (although I don't know where you are). You will end up with a far more professional product. No matter how much you work at it, a Dremel tool just won't give you the same results. Then, you have to weigh your time management versus results. Anyway, use the old one as a template, find the correct material, then let them do the rest.
The only "gotcha" is that the more brittle the material, the more difficult it is put holes in the middle of the piece. it wants to blast it to bits. There are, however, companies that can laser cut, but that's more expensive. Water-jetting is cheap. You may also have the bevel the edges too.