A local contractor runs a four axle forage wagon behind a Magnum. The first, 3rd and 4th axle are steered by a pushrod which is mounted with a toplink ball joint next to the drawbar. I think they take about 60m3 (82 cu.yard) of silage with it, and with over 3 ton drawbar weight, it gets through a reasonable wet patch too.
Another contractor put a forage box on a 430hp Ginaf 6x6 offroad truck, they take about 45m3 through allmost any terrain, clay or marsh. http://www.youtube.com/watch?v=iG6sEgUVQtQ
You can get a lot of universal gearcases from Italy or Germany: Comer, Rochling, lots of brands.
Actually, I think "Why?" is a perfectly valid question. Many things 'could' be built on a DIY basis, but don't make sense unless feaures not commonly available on production units are desired. So I will also ask- Why?