I made this diffuser for my HDS Systems flashlight. The OD at the head of my Basic 42 XRGT on the knurling is 25.9mm. The ID of this diffuser is 25.85mm, slightly smaller for a friction fit. Printed at 2mm thickness (solid fill) using Hatchbox glow in the dark PLA I get excellent diffusion with the added benefit of a glowing tip when I turn the light off!