Its a pretty straight forward technique, the display is at the bottom and in the glass cube there is a mirror at 45 degree angle facing you (you can see the mirrors edge on the side wall) which reflects the image from the display at the bottom making it look like a hologram

You can create the same effect on a smart phone screen with a video like this: https://m.youtube.com/watch?v=CIszi2Kv_lk and a some clear plastic sheet put together like this: https://www.twowaymirrors.com/wp-content/uploads/2019/11/pep...

OP: beautiful work with your surf projector!

Do you happen to have instructions on how to build the plastic sheet setup or perhaps a pointer to something that I can buy?

You can do it by cutting and folding a sheet of clear acetate or similar from a net like this: https://wp.optics.arizona.edu/oscoutreach/wp-content/uploads...

Or you could cut the four sides separately and just sellotape them together.

edit: not much help without measurements sorry - try this: https://www.holeinthewallgang.org/Customer-Content/www/CMS/f...

The required angles appear to be 54 or 126 degrees: https://data.formsbank.com/pdf_docs_html/304/3049/304991/pag...

Yup! We first started very similar with a phone and testing out the illusion then moved over to a microcontroller for a little nightstand device