What is a suitable mold material for a concrete bar top?
1/4" birch underlayment plywood (Tecply) should take that bend even without kerfs, moisture or heat. I'd probably double or triple it (without fastening together) for stiffness. 

Build your form out of lumber, then either rabbet the lumber for the plywood to achieve a flush joint, or lay plywood all the way down the 2 sides adjacent to the bend.