Thanks for the answers!
I like the look of the solution in the picture. It should be perfect for me, as the cleanout comes up in my front garden, therefore no lawn mower complications! Also I can easily control the height, so burying will not happen. I am reluctant to do anything that is metal, because it eventually corrodes and cannot be opened. I have a brass cleanout inside the house in the basement floor that was buried in the cement floor for 34 years until I finally found it. Now it cannot be opened. If that happened inside, I can just imagine what an outside cap would do.
Thanks again for this. I will buy the parts today!