Allen
I bought and put up one of these a couple of months ago.
http://www.portablegaragesandshelters.com/product/BSXG It is very sturdy and has plenty of room for my boat (21 ft malibu with illusion tower. I am very happy with it so far! I went with the gray color which looks better imo. The cover is a very strong/thick 24 mil poly material.
There is actually enough room in there with the boat that I can walk all the way around both sides to work on/ access the boat. I am going to install some lighting inside pretty soon to make it more useful. The only beef I have with it at all is the quote "average install time with two people is 3 hours"! What a freakin' joke. Me and two of my buddies spent a day and a half putting that thing up. The directions are not the best in the world...lol