Chicken Tractor question

I am going to build a chicken tractor for my juveniles to go into before I put them in with the older girls. I will also use it if I ever get a broody hen or need to isolate someone. I have been researching on here and the internet for days now. An A frame style will suit our situation the best and since our home is a modified A frame, it will kind of match the house as well.

I have noticed there are basically two ways people go. They either make a small coop at the end of the tractor or the make an "upstairs" sort of coop with the chickens able to run around underneath. I have decided on the coop at the end style so that I can make sure it is ventilated properly. I have not been able to figure out how to ventilate the "upstairs coop" kind so that the draft wouldn't blow directly on the chickens.

Now my question is a design issue. I see some who make the tractor open at either end, which is an option for me. But then I see others who have a door in the roof so to speak. I could see how that would be quite handy. However, how do it keep the door from leaking? I have seen a lot of negative posts on here about exterior nest boxes and how they leak, but it seems with a chicken tractor that problem would be even worse.

I will be building my tractor next week, weather permitting, and would like to have my design in place. Chances are no one will be living in the tractor during the winter months, but you just never know when someone might get sick or hurt, so it really needs to be a good design that could be usable year round if the need arises. If we had to use it in winter, we would cover the run part with the clear acrylic roofing panels to keep it snow and rain free and as dry in the run as possible. I would hate for any of my chickens to be in a leaky coop. If it isn't possible to adequately seal up nest boxes, it seems that the best alternative is the end doors. The roof door would be easier to make due to our limited construction skills, but we want the coop nice and dry.

Any suggestions are greatly appreciated as most of my research has turned up pictures and plans and so forth, but no real info on how to deal with this particular matter. Thank you.