I remixed this from brandonmjor's 30oz handle to fit my collection of 20oz and 30oz mugs
There are two 20oz STLs included. One for Ozark Trail and RTIC brand tumblers and one with a .5mm wider opening that works for YETI. You can always scale the STL in your slicer if you want to adjust it higher or lower on your cup.
Update: I've added a 30oz model for the larger tumblers. Same design, just slightly beefed up and larger. The 30oz STL fits the RTIC and YETI brand 30oz tumblers.
I printed this in Priline PETG (http://a.co/emnqTWA) and it came out fantastic. PETG has the right amount of flex and strength for a part like this. I used .2mm and 50% infill and used Slic3r's defaults for PET and just changed infill to 50%.
I'm not sure if this would work well in PLA, it may need to be "beefed" up for PLA since I really designed it for PETG's strength and flex. If you haven't used PETG yet, I would strongly recommend you try it.