Half-way through a recent 200km mountain bike epic, by s-works cranks started to come loose. They bolt that joins the two cranks in the middle via the Hirth joint came undone. Fortunately, I was able to tighten the cranks sufficiently using a screwdriver borrowed from a passing 4wder, but a portable tool was needed. Nothing like this exists, so it was time to make one.
My portable tool is made from a long M5 allen head bolt and the end off a T45 wrench welded together. Total weight is 24g. The bolt is about 30mm longer than what it needs to be, so now that I know this method works, I'll make another with a shorter bolt. The bolt is mild steel and the T45 tool chrome vanadium, but standard mig wire welded them together without an issue.
To make your own, get something like this Bondhus - L-wrench - Star, Long Arm, T45
If you need the crank tool just for home use where you've got a 3/8" drive handy, just get a T-45 Long Torx Socket 3/8" Drive