We have a hay rack on the top and we use it when we need it (which isn’t much anymore as we seldom take more than two horses anywhere so there’s room in the 4 horse box).
When we do use it we bag the hay in contractor bags we buy from Home Depot, load them with a front end loader, secure them with standard nylon strapping, and unload by re-creating Galileo’s experiment from the Baptistery!
We’ve carried other stuff up there in sealable plastic boxes but it’s not fun loading and unloading on the road.
But there is this if you just have to carry hay and don’t have an FEL available:
http://www.shininb.com/horse-trailer-accessories.html
https://mrtrailer.com/balebucker.htm
G.