I mounted a Dell 2-in-1 11" on top of an EMU Xboard 49 that had a Behringer UCA202 and 1/4 jacks attached with Gorilla 2 sided tape onto it for sitting in open mics playing mostly Hammond parts. Ran on batteries for over 2 hours - All I needed was a 1/4 cable from the keyboard amp to plugin in and go.
You will need to ventilate the keyboard chassis with a small fan if you put the NUC inside.
I think the Stream deck could be easily mounted on top of the keyboard, perhaps using the spring loaded tablet holder so you can remove it easily while transporting the keyboard.
This thinking is all in the don’t build your own keyboard “enclosure” box. Once you do that the project is way more complex. Christian from Germany designed a very nice custom keyboard.
Group-Buy / Collection of Ideas for CantabileKeyboard - Related - Cantabile Community