A minimalist iPad/tablet mount for "Hamilton"-style wire music stands. There's a port at the bottom in case you need to plug in a charger to the tablet, and a notch to access the home button. I used this with an iPad Air in an Apple leather case and it's a nice, secure, and snug fit, but other tablets should work fine.
The gap is 12mm, so make sure your tablet is under that thickness. The mounting hole is 11.5mm x 6.5mm which fit the music stand I thrifted for a couple of bucks, but not sure if this is a standard size. You may need to remix to suit your needs.
Warning: I would not recommend putting a tablet that is not enclosed in a case in here. They are too slippery and can easily wiggle out since there are no side supports. (I had to keep it to 120mm in length to print on my Monoprice mini.)
Edit 1/16/19 - new version with screws to tighten down the grip on the stand and the tablet