VAR Chainwheel Nut Tool PE-35400
If you've used one of those flat bent metal tools to hold the sleeved nut holding your chainrings on you'll know how fiddly they are to hold in place. (Or is it just me?)
Anyway, here is a proper screwdriver shaped tool to make things a lot easier. Unfortunately cannot be used if you have one of those chainsets where one of the arms is hidden behind the crank. We have cranks with all five arms visible if this is you.