IMO< the answer is NO, they will NOT fiot ina an 11 ft enclosed clam shell
and the reason is, the hood of the sled will hit the cover as will the tail of the sled, as these clam shells normally have some slop to the covers that take up space, as this is Not an open deck trailer when there is NO worry about the hood contacting anything!
every model will also be different, so, its not just as easy to say clam shell and 8.5x11 ft being the same as ALL of them
even a 8.5x12 might NOT fit them pending the design of the cover being used!
it will be needed to look really CLOSE at things before buying IMO