I thought it might be interesting to just have a thread where everyone could add any DIY tip or tricks that they had stumbled upon.
Scotch Magic Tape...obvious one but cleans up any beam where throw isn't needed.
With aspherics either fixed or "zoomies" it can clean up some of the artifacts if you use it on the back of the lens as a mask...use four pieced to mask off the lens leaving a large square unmasked in the middle.
Use black spray paint and a brush (spray the paint initially on a piece of scrap wood) and paint out the Cree ring and anything else that might reflect light when you are working on an aspheric of any kind.
O-rings are hard to find in kit form that cover all of the common flashlight sizes. ACE hardware generally has a decent selection of individual o-rings in a flat file container somewhere in the store.
You can also buy o-ring kits for about $15 online that come in a large plastic box and are for cars really and are too thick for flashlights in most cases but the diameters are there. When you have o-rings like this you find ways to use them. Grips on flashlight bodies, spacers when you mod a light and the head doesn't screw on completely.
I recently used one to help focus an aspheric. Since the o-ring was thick and the aspheric wasn't perfectly focused the thickness of the o-ring moved the focal point in the right direction (too far) and by tightening a retaining ring it adjusted it back (by compressing the o-ring) to the correct focal point.
Light grinding or working with plastics...a small random orbiter sander works well and is small.
If you need custom plastic parts that are basic you may be able to make something yourself with PVC from the hardware store. You can buy a "coupler" (I think that's the correct term) to connect two pieces of 3" PVC pipe to each other.This is a $3 part available anywhere. Cut it in 4ths lengthwise and you have 4 pieces of sturdy plastic. Cut it with a hacksaw, sand it with any sander. You can even shape it with a sander.
Mold it, within reason, by putting it on a cookie sheet in your oven at 250 degrees F for 20 seconds or so and take it out (it's now like cheese) mold it and drop it in water...it's now hard.
Never throw any cheap lights away as they are a good source of parts later :)
Forward clickies are available to Shiningbeam for $3.
Needle nose pliers take apart most any flashlight :)
Although I have some soldering equipment I've not done any regarding flashlights (too tedious) but I may at some point in the future.
One skill I've had to develop therefore is to try to pick the right host in the first place. With all of the choices out there it's possible to pick something that doesn't have to be modified so extensively if you pick the right item initially.
With cheap items from China and long shipping times I'll cover my bases and order several choices hoping that in the end one will work out and the others may supply parts for the final solution.
That's all I have for now. What are some other tips from those who know much more about DIY than I do?