Something simple that works well for me is this:
These extras I have are for sale BTW /shameless plug
Copper disc soldered onto the driver board, increases current flow a bit and greatly increases heat sinking, that contact plate gets hot after some use!
Simple .80 cent copper washer (ring) works best for me, button tops or flat tops, no problem. (The wider ones on the right)
Don't recall details, but didn't O-L find out a long time ago that pure copper has problems when using it for direct electrical contact? He had problems with lights after some time because of oxidation, and it' cant even be seen. So you would have to treat it periodically? Maybe in this use, it's not a problem because of the constant contact with the battery + ends that would grind the oxidation off effectively?
O-L made a vid to show how to treat the surface contacts - can't recall what he recommended - light sandpaper, or something like NO-OX-ID would probably work.
Pure copper is not good in sliding (?). I want to say that if battery top is sliding over this ring many times friction forces can take of lots of copper.
Oxidation is not a problem, russian eletcrical fabrics still making lots of uncoated contacts from pure copper and simple way to avoid any oxidation is using special spray, I prefer liqui-moly 3110.
This is my thinking, it could also be made our of brass. It would save some cost as well and would not make a difference in performance.
However, ground-off oxidation doesn't magically disappear. Some will remain between the contact and battery, increasing resistance/decreasing current flow. I don't know if there will be enough to make an actual difference, though.
Maybe it would be easier to machine if the fins were a spiral, like threads, but maybe still with the depth varying. That would save the tool having to move in and out for every fin.
My Nitecore TM26 uses a brass ring, & it is as good as new after more than 3 years of use. 
Best Regards,
George
Do you have a China shop for buying these?
This may be a size
http://www.uxcell.com/30mm-40mm-15mm-metric-ring-shape-copper-flat-washer-pcs-p-687105.html
This may be too big
OK OK sorry for asking (I’ll take a srk driver to the hardware store next time to find a disc for my RMM V2 drivers) didn’t mean to detail the thread 
Not sure on electrical performance, but Zinc-plated steel looks nice and is cheap and easy to find.
I did it to my SP03 Here
electrical conductivity of copper is 100,silver 106,iron 17.
Yikes :person_facepalming: Actually steel is listedd as “3-15”, even the zinc is only 27!
What material are the battery contacts though?
Pure nickel
I have 3-channel PWM working in the bistro-tripledown firmware, and the ramp calculator supports 3+ channels. However, the third channel runs at 31 kHz and I don’t know how to slow it down. And, as you noted, it would eat a pin currently being used for other purposes.
It can make the mid-high modes more efficient, but it’s a pretty big tradeoff due to the added driver cost and complexity, and probably eliminates the indicator light. It also tends to put a second elbow in the ramp and makes the driver even less friendly to changes in the number or type of emitters.
Usually get them at the Parker/Exotic store- we have one in our plant. But you can get them online too.They are actually copper hydraulic sealing rings (crush washers) - they come in various sizes and thickness. I use them for making driver adapters, 26650 battery tube spacers, heat sinking drivers and contact rings for SRK/M6 builds. Shouldn’t be a problem to get in China.