Not sure what size/shape the "tailgate opening" is in the M416 you're looking at, but starting at the post in this link are some plans and step-by-step instructions for building a tailgate, maybe this will give you some ideas: http://tventuring.com/trailerforum/thread-119-post-3824.html#pid3824. It's designed to be a fairly simple DIY project, just a little welding required, no complicated bending, everything is made from stock shapes...
All m416 trailers originally had 4 welded sides, many had the backs removed or converted into a tailgate after life in the military (and a few were removed in the military).