I made these simple LED light diffusers for 3mm and 5mm LEDs.
No glue is needed, just push the diffusers onto the LEDs, and they should fit snugly enough not to fall off.
I suggest printing in transparent PLA/ABS, but I guess nice effects can be achieved with other colors as well, even if not transparent. For my project, I needed them to be transparent.
The last photo shows the project I made them for, a 4x4x4 touch screen controlled LED cube.
Print with 0.2mm layers, better on a raft, given the very tiny footprint the diffusers have.
I suggest transparent PLA filament.