I finished my Android/XBoard project.
The back plate was scavenged from 2007 TV. I used 3M 30lb double sided tape to attach it plus 3 screws. I cut little 1/4" by 1 1/2" plates and epoxied them to the inside, then drilled a 5/16" hole and used self tapping screws. When I screwed in I held the little plate with a pliers.
dcs_xboard_phone_mount_back_plate_2 by dsteinschneider, on Flickr
The 4mm thumbscrews are from PC's. The metal brackets were 1/16 thick corner braces. These also will hold a Dell 11" 2 in 1 laptop. That's why the slotted bracket on the right sticks out - the laptop is narrower than the NanoKontrol2. I'm holding the USB C cable to the plate with a neodymium magnet scavenged from a hard drive. You would think that would somehow impair the USB C OTG connection but I've played this for hours with no noticeable issue. The phone isn't in place because I needed it to take these photos The cable tie down just below the USB C OTG connector is just to keep the USB C connection straight.
dcs_xboard_phone_mount by dsteinschneider, on Flickr
Here is a photo of the underside of the Xboard 49 top. The threaded nut plates are for surfboard fins. The are attached with 3M 30lb double sided tape.
dcs_xboard_surf_fin_plates by dsteinschneider, on Flickr
I have assigned the top row of the NanoKontrol2 buttons to the Vibrato and Percussion Tablets. The knobs control Volume, Drive, KeyClick, Bass, Treble and Reverb - the right most one controls drawbar 9.
Thanks to Max Tempia for inspiring this build