I am using the recessed ones that you flip up. They work great! They do come in 2 sizes, depending on the spacing of your cross beams on your trailer. 18" or 24". I went with the 18s so I had more room to move forward or back when installing. Currently I have 2 installed, I will get 2 more, now that I know they work. I paid 33 plus 14 in shipping per unit. Found them on the auction site. Some trailer place in Kentucky had them. I highly recommend them.
I have used the removable pingels too. Don't like the fact that once you remove them, you still have to put them somewhere, and then, when you are ready to reinstall, you need to find them. For us old guys, this can be a challenge!