The diffuser is made from a piece of 5/8" clear acrylic rod stock. For details, please refer to the provided file diffuser_single_shop_drawing.pdf for clarification of the steps below.

The rod is turned down to a diameter of 14mm in a lathe using either a 3-jaw or a universal chuck. You probably want to use a properly profiled HSS (high-speed steel) lathe bit for plastic. I used a triangular carbide cutter in a holder, instead, because it: a) was there, b) was sharp, c) had a nice radius, and d) it saved me re-grinding one of the old (beater) HSS tool bits until I was happy with its edge and profile.

The nose was then angled and the entire machined end is given a quick sand with 400grit silicon carbide wet/dry sandpaper, soaked with water-based cutting fluid, to frost the finish and radius the corners at the nose end. The neck region is then turned down to 10mm diameter, and parted-off so as to leave the neck just over 8mm long.

The rod is now wrapped in a *single layer* of note paper and put into the chuck with the parting surface exposed. Don't over tighten the chuck -- it will leave dents in the nice, frosted finish you've made. The neck can now be turned down to 8mm long using end-facing techniques.

The faced end needs to have a starting dimple put in with a centre drill. After that; the 5.2mm (I think I used a 13/64" bit) is hole bored into the end to a depth of approximately 8.5mm. This hole is then internally polished to a nice diffuse frost, using a strip of the 400grit sandpaper wrapped around a piece of 1/8" drill rod. Remember to apply cutting fluid regularly to keep the sandpaper from clogging.

Test fit you LED and your base *before* removing from the chuck. Modify as required to achieve the fit you desire.

Wash the part with soap and water and leave to air-dry.

PS Be kind - clean the lathe after you're done ;)