This is a simple dome-shaped diffuser for a common type of LED spot with 3 LEDs and a 50 mm outside diameter. It comes in a smooth and low-poly version. The latter may be somewhat easier to print. Supports should not be necessary, but you will definitely need adequate cooling to print the large internal bridges and steep overhangs. A brim may also be a good idea, although it should not be necessary with good adhesion.

If it is too tight or loose, scale it up or down by a few fractions of a percent.