How thick does an acrylic surface for a DI setup have to be. I have a 24 by 24 box and am putting a sheet of acrylic, glass, whatever on top. How thick does it have to be, wouldn’t a thin one be better?

As thin as you like really.  Just bear in mind that when you press on it you don’t want the surface to flex as this will subtly distort the image being projected.  Also it wont be as resilient to wear and tear.  6mm up i’d say if using acrylic. i take it thats 24 x 24 cm or is it inches?