Welcome to the AirForums! Always glad to see a fellow Interstate owner!
I personally do without a cover, and haven't missed having one.
There is no such thing as a cover specifically for Interstates, and a plain-jane Sprinter cover from somebody like Camping World will have a loose fit. It has to, because the fittings on the roof will vary from Airstream to Winnebago to Roadtrek to Great West to whatever. Even within one RV line, the fittings on the roof will vary by model year. One-size-fits-all really means "one-size-fits-nothing."
On the other hand, you may be able to find a sailmaker or tentmaker who is willing to make a custom-fitted one for a price, if you really want one.
Engineering: Finding complex solutions to simple problems you didn't even know you had.