I use an external keyboard for my work computer, so really I'm only using its built-in display.  Specifically, it's designed for a 2019 Macbook Pro.
 

I figured it would be nice if I could set stuff on its keyboard & trackpad without accidentally pressing buttons.  This design, at the moment, doesn't involve an actual shelf.  I designed these clips to hold an 1/8" thick basswood panel.

I may redesign to make a modular snap-together printable shelf, if that'd be interesting, let me know.

If you'd like to do that yourself, or adapt it to another laptop, go ahead and remix!  I made it in OnShape: https://cad.onshape.com/documents/c282a5cd8c2fbdb6785abb1f/w/e409b65abfe86f6bd07826d9/e/2dc19cdc83fa05ecf791d2fc?renderMode=0&uiState=678f077eb30bc30b59f0ec09