I actually just did a very similar setup a couple weekends ago using D rings. Harbor Freight had D rings on sale for $4 apiece so I bought 8 of them and made spots for all 4 corners. I got tired of fraying straps on the corners of the trailer using the stake pockets.
Just make sure that if you're attaching the D rings to lumber only that you use an oversized washer or fashion a plate so that the weight distributes across the wood on the bottom side. If not, you may pull the ring up through the wood if you get enough force applied to the ring.