Ok, both of these were built many moons ago, but you can an idea of how to go about it. The first one is PVC pipe, with the fenders made from thin alum flashing cut and bent into shape. The axles and susp was from some AMT trailer. The arms are made from brass tube.
This one was made from the bottom half of a tanker, and the frame of the MPC/AMT dump trailer kit. Its way too long and unrealistic, but wasnt bad at the time. I built this about 25 years ago and was probably one of my first kitbash/scratchbuild projects.