To keep the combined knowledge and experience of this group in one thread, please see this Fudman/Hooray! post where the first home-made counterhold tool broke due (most likely) to the steel and thickness used.
Here's the post mortem tribal knowledge learned:
"My original fan clutch "tool" was jury rigged from the flat metal bar stock that was used to lock my tool chest. It was a few mm thick and of relatively soft steel. I had the shop boys at work drill the holes for me and figured it was strong enough. NOT! The small section between the curved arc cutout and the oval hole snapped as soon as we torqued the 32mm wrench. Sorry, no picture as I left the "tool" at the machine shop.
Plan B was to borrow Chris' (Neversaynever) tool but he wasn't home. Plan C was to modify an old section of garage door holder I had saved after installing a new garage door opener. This piece of flat stock was about 3X thicker and of harder steel. We were very lucky to find a machinest willing to do the work on a late SAT AM. The machinest drilled the holes and I carved out the arc using a grinder. Now this one is a TOOL. There is NO WAY this tool will break."