I've used a wrench. Doesn't work too well in my never-to-be-humble opinion.
Make a 'tool'. Consists of a piece of sheet metal a bit wider than the fork tube with a slot cut into it the width of the rod, as deep as the fork tube radius + 1/2" or so.
It takes up less 'length' than a wrench and stays put whilst you mess with them longer-and-soon-to-be-appreciated XR springs! ;)
While I'm sure the description is perfectly clear, included is a trademarked template for this complicated tool genesis. (yeah...sometimes I just kill myself :) )
BTW..3-5mm is fine on the preload. To get that small you will have to lose one of the oem centering washers...the top one.