For those hard to reach plugs on a flat head with aluminum heads. I bought a shower valve tool that had one end (29/32") that was a close fit for the 7/8" (28/32") hex of a Champion #D16 spark plug.
I sawed the 4" long tool in half, then pressed 5/8" nut (15/16" hex) half way into the sawed end. When finished, the tool fits in under the oil tank and slips onto the plug. Use a 15/16 wrench on it.
Sparkplug (.875 Hex) Socket .jpg