I have a number of instruments and wanted to know if the Tonewood Amp has ever been applied to a Cajon?
Second, I've seen the amp on a baritone uke and wanted to know about it being used on a Guitalele (yamaha).
I think using it on a Cajon might cause some issues due to the thickness of the sides. I wouldn't recommend placing it on the snare side either. However, I haven't tried it so I cannot say for sure.
As for your Guitalele, placement might be trickier on a smaller body, but it should work as long as you have a connected pickup, and can fit the X-brace inside the soundhole.