I have a cover from a 96 Impala SS (!) that a friend gave me. I put it on the car for fun one day and it actually fit OK, so I take it with me on trips with the car if I think there is a chance it could rain. My car is a race tail with a factory high-mount wing.
Pete- you may have seen it on the car at LS Fest, as I had it on Saturday night when the storm blew through. With the sides staked to the ground, it stayed on all night through some heavy wind and rain.
That said, I would be in for a custom-fit cover. But there are so many variations of the SLC (street tail, street tail with a wing, race tail with the low wing setup, race tail with the high wing setup, Le Mans, etc) that a truly custom fit is probably just that- a cover made for one specific car.
Apparently, there is at least one company that will scan your car and create a custom cover based on the scan. I've thought about that, but the cost is pretty high (well over $1,000, IIRC).