You want to build a four wheel wagon for your Cub? Or a regular sized one?
You can buy a pretty nice frame for a 4 wheel wagon through Northern Tool. Frame is extendable from 49" to 78". $284, and to my zip code shipping is about $75, so not a bad deal. You still have to build the bed.
http://www.northerntool.com/shop/too...ct_47661_47661
If nothing else, use the pics as a pattern to build your own. You don't need axles, as you can see, the whole running gear is fabricated.