Over the Tailpiece:
If you find a chinrest you like, but it’s only made for violin, it can be easily used on viola if longer barrels are purchased. https://goo.gl/Ed3H9S
Custom made/fitted chinrests:
Many luthiers can work with you to create a chinrest that fits you. Talk to your favorite luthier and discuss your options.
Chinrest covers: Gelrest, Chin Cozy, Kinder Chinder, Stradpad