I just did this last week.
first remember the threads are reversed.
there is a special tool that helps hold the pulley nuts so you can keep it still, but like you i didn't have tool
What ever you try to stick in there it has to be thin cause you can't get the big 32mm wrench on the fan nut, if not.
I ended up being able to wedge a small thin bicycle wrench in there. the mouth of the wrench was on one pulley nut and the shoulder of the wrench caught another, the end of the wrench rested on a belt and other pulley so i put a piece of wood there so as not to mar the belt.
took a few tries as it was a balancing act but finally it locked things up enough to loosen the fan nut.
oh and by the way it was not a 10mm bicycle wrench holding one pulley nut, it doesnt work it just unscrews. it was a 13 or 14mm wrench and the trick was to get the mouth on the one nut and the shoulder on another
Good luck