I couldn't find a cover either for my '64 with dual antenna's that already had the slits. So, I bought one and carefully made the incisions. I undid the antenna and pushed it through to the trunk. Put the cover on and you'll feel the whole. I made a small X and pushed the antenna mass through the hole just to make sure. Once satisfied, I made the hole bigger and round to accommodate the antenna mass nut. Nice fit. Just go slow. Cut a little at a time.
As for car covers, I have 5 California Car Covers. Great quality. Great fit. Went to buy another for my '64 SS and couldn't believe the price. $275.00 for the Noah. This forced me to look elsewhere. There are plenty of car covers on line. Narrowed my search to 2 then to 1. I then bought the cover from them for about$120.00 less. Its been 2 years. Held up great. No problems. I would buy again from them. Sorry, I don't recall the name. Just remember this when buying a car cover: compare apples to apples. If you want a Noah from Cal Car Cover, look at their specs that they advertise. If you find another on line, compare the specs of both; layers of fabric, fabric composition, lined or not, water repellent, warranty, or anything else that might be important to you. Cal Car Cover make a great cover, but unfortunately for them and myself, they priced themselves out of the market, Carmine.