I don't know anything about the carriers already out there so I'll be no help there but I just wanted to say that there are some patterns out there for carriers, since you mentioned that.
Simplicity has some as well as butterick
Here's a link to show you what butterick has...
http://www.smartcart.com/1sew/cgi/searchindex.cgi
and I don't know how to actuallylink to the simplicity ones, but you can goto their website
www.simplicity.com and search for the numbers...4716 and 4749
I've made, well halfway I quit making it, the 4716 carrier. It isn't too hard at all it is an open carrier, it doesn't have a top at all.
There may be more out there too I don't know.