Unfortunately unless you have access to a flexible material you can extrude on your 3D Printer, you’re going to have to get creative. That’s what Robogrrl did. The concept is simple (the execution is a little more complex however). Use your 3D Printer to print the pieces you want joined by a flexible member, then 3D print a mould of the flexible link. Pack sugru into the mould. Boom!
The complicated part is designing the hub and mould. Luckily Robogrrl has a pretty in-depth tutorial on her process. She’s using inventor for her design. The end result is pretty astounding!
Full tutorial is up on her site.