Near the 5/8 inch arbor hole you see two flat washers. Their outside diameter is right at 5/8 inch. Their inside diameter is just over 1/4 inch. They will make a nice center for the blade to hold it in place.
Step 1: Plywood square
Step 2: Attach a screw, the washers, and a nut
Step 3: Make a small square of plywood
Step 4: All done
I could make my blade keeper more decorative by giving it eight sides or rounding edges, even painting it. But, it works well as it is, and my blade is protected.