Looks good Travis. Is one of those mine!!
It looks like that plastic is easy to use. Rims look good as well.
I can see one thing though. I am not sure if you overlooked this or just want to do something different. Generally on these trailers the 4 frame rails sit flush with the wood deck. I noticed that the cross members sit flush with the frame rails so when you deck the trailer the frame rails are going to either be covered or sit lower than the deck. If it were me I would cut the cross members down the same amount as the wood decking material.
Just my opinion. You might have something else in mind.
Have you figured out the sag issue yet, you mentioned making a tension system.
-Mike