I'm gonna make a semi-uninformed guess that a standard non-form-fitting bass case is going to have way to much play to fit the longhorn. It will more than likely go inside, but it'll move all over the place.
I've been holding out to buy myself an official case, but still haven't yet. I just carry mine in a padded guitar gig-bag which fits it perfectly.
The eBay link that Denis posted is the most consistent place I've found Longhorn cases.
Occasionally you see one of the official ones pop up on eBay, but they generally sell for more than the Longhorn itself costs as they are becoming more rare.
I have often wondered if the Dano longhorn would fit in the Hondo Longhorn case.
Someone sent me the inside dimensions of an original Danelectro case, I'll try to post them tomorrow.
