Trailer lights are always a problem. They are usually cheaply and corrode badly in a few years. Mounting any light on the back of a snowmobile trailer is probably going to end in disappointment. The reason the mfr. doesn't mount them there is they would probably be knocked of when you drop the bed to unload the sleds. If you mount them high enough that they won't contact the ground when you drop the bed, the sleds will knock them off when you load them. Or the sleds will hit them and demolish the lights when you load them back on to the trailer. Unplugging any trailer light before you back in to the water is a pretty sound practice.
Avoid the Peterson brand of lights found everywhere, they corrode quickly and the spring contacts lose tension quickly, both situations leaving you in the dark. Westbar is quite a bit better, not too hard to find either. My best experience has been with those big round sealed units that mount on a stalk, but they are a pain to mount as you need to drill a big hole- maybe 3/4" or bigger. They also stick out quite a ways from the trailer, maybe 5 or 6 inches. Something to consider when you start with something as wide as a snowmobile trailer.