I am getting ready to mount a old 4 ft john deere plow off off a 318 tractor on a machine and am in the same boat. I mentioned before about how the poly's would work, and/or if they make any that small without ordering a full size one and cutting it in half.
This may sound funny, but I have the idea of maybe trying a piece of 'TREX' lumber. Its the fake, plastic wood reinforced with fiberglass. We have A few 2x4's lying around, so I might give it shot. It seems like it might hold up, and does not splinter like wood. My only concern is whether or not it will leave marks and whether it will wear well. One 2X4X8 is around $25, so its expensive, but maybe it work well for an application like a garden tractor plow. I'll tell you how it works as I am getting ready to mount the plow and set the hydraulics up on my machine for the possible 'blizzard' we may get here on Sat.