1/10 scale 4' x 8' trailer frame with working leaf spring suspension and axle tube.
Axle tube is designed to accept two .375" x .188" x .125" bearings, one on each end and a 10-24 x 7.625" threaded rod as the axle. Capture the bearings on each end with a 10-24 nylock nut. This will allow you to attach 1.9" crawler wheels and tires using a 12mm hex adapter.
I have a 1:1 drawing available to cut a bed floor out of .03 plastic sheet with the stake hole locations.