I have an idea, what if you used a suction cup mount? Take a look at this.

https://www.amazon.com/ARTIX-Adjustable-Multi-Angle-Aluminum-Suction/dp/B017WNRSVG/ref=mp_s_a_1_fkmrnull_1?keywords=ARTIX+360+Degree+Adjustable+Multi-Angle+Phone+Tablet+Aluminum+Stand%2C+Holder+With+Suction+Cup+Secure+Base+for+iPhone+iPad+Samsung+Android+Kindle+up+to+11+Inches+For+use+in+Car%2C+Home%2C+Desk%2C+Office+Black&qid=1556311989&s=gateway&sr=8-1-fkmrnull

This is considering you have a sun roof.... But then you may want to cover up the sun glare... This would involve some innovative ideas for optimum viewing experience!

There are also gooseneck iPad mounts that are clamp on! This might be the best easy solution