I use one of these on my 12.9” iPad Pro with its case on. I find this mount to be far more secure than all the other ones I’ve tried.
Neewer 7-14 inches Adjustable Tablet Holder Mount with 360 Degree Swivel Clamp for Connecting with Microphone Stand, Compatible with iPad, iPad Pro, iPad Air, Google Nexus Samsung Galaxy https://www.amazon.com/dp/B07D5TSC7C/ref=cm_sw_r_cp_api_i_ACepEb761C837