OK, here's the full situation. My MCs have an interdimensional gate technology that allows them to travel between alternate earths. They can't "tune in" to any world they want, like stargate. It's more like a subway. A connects to B which connects to C, etc. And it connects to the same geographical location on the next earth. Some earths are not friendly (one never experienced the Yucatan impactor). So I'm visualizing them setting up a series of underground containers with a gate at each end. As you walk through them, you're moving between realities. Obviously you also need an exit from the container when you get to the earth that you want, but that's more of a detail.
For ventilation, I can just use some kind of forced-air system at every 3rd container or something.
The other problem - and I just thought of this as I was typing -- is that materials have to come through the gates, so I guess I can't use a completed structure like a shiping container. I should probably be thinking of this more like mining, where you're digging shafts and bracing them.
So, the 'moving between realities' means you don't need the containers to be living quarters, etc. Just a means of separating the 'gates'? And the containers don't need to be a particular size or length, (the alternate dimensions aren't actually 20 or 40 feet apart) as they're just a gate-holder and sort of a placemarker for the traveler?
How big is this gate? How much space does it take up? I'm going to assume that it's 'wall-thick', as in: it wouldn't be any wider than a shipping container wall. How high, how wide? Because the smaller the space you need, the stronger you can make it.
So, you're looking at a space big enough to hold a couple of people for a short length of time, with a third door as an exit?
How about elevator cars? You can get them with doors front-and-back for going into the 'back rooms' of buildings, and they have those emergency escapes in the ceilings (or at least they used to.) I assume they're sturdy enough to take being hauled up and down, with a weight of people on them (I don't know how they handle impact, when the brakes fail or whatever.)
* I know you don't want to have to do construction, but if the size of the gate is the constraining factor, concrete block construction with rebar reinforcement sounds like the way to go.