I think I found the ideal pin:
3mm clevis pin, 20mm length in stainless steel. (16mm is ~1mm too short). Cost is acceptable (23 pence each) but I have only found a UK supplier.
This gives 3.35mm of clearance. Thus, 3 washers on each side (0.5mm for each M3 washer), would give us a good small clearance of 0.35mm.
Next job: find a North American supplier. Boneham's has a USA subsidiary but no pricing:
It might be easier (but more expensive) to find M3 shoulder bolts.
OLD3mm drive shafts seem to be common on RC cars.
The length should be enough for 10mm extrustion + 2*2mm bracket + 2mm head and 2mm c-lip locator. So we are looking for ~17-18mm pin length.
Tamiya offers an RC car 3x18mm shaft for 1.25$, all stainless:
It would be fairly easy to make these pins from brass (as a bushing) or stainless steel (inside a bearing) by turning on a lathe, however this entails a much larger cost.
Polulu offers long shafts in a Tamiya kit at 5.75:
Also Headsuprc.com has a big list of different shaft sizes available:
HPI RC cars seem to use 3x17mm shafts which may be acceptable.
Technobots in the EU offers much cheaper and longer 3mm shafts and even has technical details available regarding surface hardness (Over 180B). In the UK, the "silver steel" they mention is actually tempered steel drill rod. So it would be possible to buy 3mm drill blanks in NA and use those.
There is a good discussion on machining circlip grooves into 3mm SS pins on CR4:
3mm circlips are available at Amainhobbies.com:
Another alternative which allows non-grooved shafts to be used (much cheaper and easier to source) are these 3mm grip rings: