This is a VESA-mount adapter for the big iPad Pro, so that you can stick it on a monitor arm and use it as a second monitor. I gave it higer-than-usual infill: I think I used 40%. You don't want to go too high on the infill, because it needs to flex a bit to “grab” the iPad.
It was designed for my A1652 iPad Pro (12.9-inch) from 2015.
The screw holes are intended for M3 screws. You will need washers on the back because most VESA mounts have pretty large holes. The holes are 5mm deep, and the screws are not intended to come all the way through. If you want to use longer screws (and nuts, etc) you could drill them through. There's a bit of room between the mount and the iPad, but not a lot. I think I used 8mm screws (I don't remember!).
The author marked this model as their own original creation.