This is a 1:10 scale single wheel trailer chassis based on the classic Allstate trailer. It can be fitted with a good amount of custom bodies. It's right at home mounted on 1:10 scale on road cars. A little small for a crawler, but can work if the mounting posts are turned upside down.
Instructions are included on the images.
You'll need one of each part, 3 11x4x5 (Traxxas) bearings, 1 3x15, 2 3x28, 4 3x10 and a custom sized machine screws. And a 3mm tap.