I had a Duke 10 for awhile and also failed to find a suitable hard case. But as I posted here back in December, the Duke 10 does fit pretty well in the Reunion Blues RBX Tenor Gig Bag. Not a hard case, but much better than the lightweight bag that comes with the Duke.
I bought one of these from Alan for my Ludwig Wendell Hall. Top-notch case and good value for money, and a perfect fit for my WH. It appears to be too shot for the Duke mentioned, though. You may want to check with Gold Tone. They make a wide variety of banjo ukes and may have a case to fit.