The easiest thing to do is file down the sides of the larger prong. This will not harm the plug, and will not take very long as the prongs are made of brass. Otherwize, look for another adapter; older adapters have prongs of the same size. That's one reason it's referred to as a "cheater plug"...

Just go down to the local hardware store and buy a grounding plug adapter. One end accepts a standard three prong plug, the opposite end has dual blades with either a grounding wire or grounding lug. Plug your cord in and flip the plug for your proper polarity. Place the grounding wire under the...