After trying a few generic online models I designed a holder that would work with newer iPads that are 6mm thick. So far I've been using it without gluing any parts together, they are snug enough to hold it, although a bit of sticky tape/rubber bands might give you more confidence. The one key thing for my use is two bits of foam tape where the holder “pinches” the iPad. This secures the iPad firmly, yet it can still be extracted without much force.