In looking at the "clam-shell" style trailers, you have a couple choices-an aluminum or fiberglass cap. According to the manufacturers, they weigh the same, just depends on preference-aluminum can damage more easily than the fiberglass but is "wash & go", probably would need to wash and wax the fiberglass one occasionally, more so if left outside year-round.
Here's the fiberglass cover that can be put on most 12' trailers:
http://fibertechservicesinc.com/snowmobile_trailers_AH12.php
Here's a link to Triton:
http://www.tritontrailers.com/products/Models.aspx?CatID=25&SubCatID=27
I, personally have had a Triton w/ their enclosure for over 10 years with no problems. Previous to that a Sled Bed open-bed which also was a fine trailer.
There are other brand trailers out there, but Triton has a nice package and available options, and pricing isn't too bad and there seems to be quite a few dealers who sell them, which in case you were to need anything, is a good thing. I would go with the Elite model if you choose a Triton.
Floe, from what I have seen is a well-built trailer, but not inexpensive.