When I was reading up on the procedure to replace the water pump, I noticed that some were having difficulty in holding the water pump pulley, in order to remove it's 3 bolts. It only took a couple of minutes to throw together a home made spanner wrench. It doesn't have to be exact at all. I used an old scrap of aluminum. Even a piece of 1"x2" length of wood would work. The two bolts on the end of this spanner are inserted into the different holes in the front of the pulley to prohibit pulley rotation when you need to loosen/tighten the three bolts that hold the pulley to the water pump. (Whew!) Hope this may help some.
Here's the professional tool. I've used it for years to hold cam, fan. crankshaft and water pump pulleys. It's great for changing timing belts.
With a simple modification, the OP could make an adjustable tool for a few bucks. Or simply drill more holes in his bar stock. SCH-I535
