I will be making one on my build. I'm keeping a 5' width with a cross ways bed so I can stand up. So being 6'2" tall I need somewhere for my feet to go. My plan is to makw a lid open like a baggage door that is hinged on the top. Then slide out a 3 sided drawer and drop the lid down as a top. the drawer will have a slight angle so rain runs off. I will use draw latches to hold the top to the sides. I have found a set of 100lb slides that have a locking position on it. If these are placed just right, when I pull out the drawer the slides will apply pressure to the gasket and then lock in place. It should work.
It will require a special mattress because the bed will have to fold to allow the drawer to invade the bed. I'm going on a 3 week trip so this will not be worked on till Late June or July.
Here is a picture of what I am doing. There is a build thread started if you want more pictures.