Sounds odd to have that much trouble finding a plug. If all else fails, dig out around the pipe, take a hacksaw and cut down below the whole top section (the fitting with the cap) until you have just the 3 or 4 inch pipe sticking up, then go buy you a whole new cleanout adapter and plug. Shouldn't cost but $5. I would advise against putting it at ground level, but it's his yard. The ones at ground level tend to get covered up over the years somehow and then the next homeowner is going around cursing trying to find a cleanout. Or - it gets mowed over because it's "hiding". I leave mine exposed a few inches.