Ed a friend of mine was a Red Neck trailer part dealer and made some trailers for a while. He built one for a guy that was configured like drawing below. Flush mount (counter sunk) "D"rings, in 4" and 2". It was over kill with the amount of rings, but the guy said he was tired of not having enough. The 2" ones were located next to the very edge of the trailer (and down the middle). He said that was the handiest trailer he ever had.
As far as the strap down, we used the D rings and always criss crossed the ratchet straps frt & rear. (LF car to RF trailer, RF car to LF trailer, etc.) Never had one slide to the side anyway.