The photos I have of the Sd.Ah.51 trailer are not good enough to use for scratch building. The trailer is basically just a box mounted on the same trailer used to tow the 20mm Flak 30. There appears to be more than one variant, at least from what I can see in the photos. There is a 1/72th scale one made by TP Models, maybe you could use this and scale it up. Try Google image search for "Sd. Ah. 51", you'll find at least one image.