Harwood makes some nice add on cowl pieces. They're mostly taylord to GM products and Pro street style scoops though. You could check out what they've got and see if anything tickles you fancy though. I'm with SaleenGT2001 though, for the price of a complete cowl hood now-a-days, it's almost not even worth it to do all of the measuring, cutting and body work to add an aftermarket one to your stock hood.
I agree with Gearbanger agreeing with SaleenGT2001 about being right. The price of the complete hood can't be much more than an add-on cowl nowadays. You'll be happier you did in the long run...