Not only is there room for a small person under the bed, but you can add more beds above the current one and make it a full holiday camper <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

About sandwich and steel.. When doing tilt trailers for Tornados I have always heard that the sandwich Tornado hulls are stiffer than the trailer frame. Hence you try to support the boat by strapping only parts of the crossbeams to the trailer. No use stiffening the trailer with your boat. I guess your sandwich is stiffer than the trailer frame, hence the stress cracks?

I see several advantages to using a large box, and some disadvantages. One advantage is the ability to put the whole boat into a dry, secure, clean, trailer when on the road or during winter (unless you decide to sail trough the winter, like someone I know..). Another is balance of the trailer and windage. You also gain a lot more room. If you can park the trailer at the club, you have your own dressing room and storage. You can also have a small workshop in the back of the trailer. A "box" trailer also be used for domestic chores like bringing home new furniture, firewood etc. Major selling point with the wife..
Disadvantages is size, weight, cost, problems with tall items and possibly fuel efficiency. Your trailer dont exactly look aerodynamically streamlined, but I dont know if a rectangular box would be better or worse..

Did you buy a 'clean' boat trailer which you buildt on, or did you build the trailers frame as well?

Upon second reading the technical solutions and installations look really good. You are not an engineer or similar by trade?