Raced with an open trailer for many, many years. Conditions required we go with an open trailer.

There are several good suggestions already posted----but there is more to consider(or dream about). For example, we built our trailer from square tubing---which we welded air tight---and used the trailer frame as an air tank.
Another, we built our trailer to the max width(102") and by building our fenders---allowed us to build a very low trailer---to ease loading.
Another, our ramps telescoped up into the floor of the trailer.

My point is, use existing designs as a guide---but custom build it to fit your needs!