OK, as an engineer (OK, really just a designer), I need to throw in my 2 cents.
The thing about a big round opening is that the distance the force field would need to span would be about 4-5 times as wide, with a lot larger total area as well. This would probably be much weaker in the middle, but no guarantees about that. So I think the rectangular slot works as a good design. But who says it needs to rotate with the station? It could be stationary.
But the whole concept of force fields and and atmosphere inside the station is ridiculous. There should be a vacuum inside the station (as well as unoccupied areas of ships, like cargo holds and hard point housings). That goes for the hangers, too. You can use a docking tube and/or airlocks to enter the occupied areas of ships and stations. Otherwise you can walk around in your vac suit. Since we have shields, I could see a slot shield to keep debris out, but also armored blast doors.
And to keep everything moving smoothly, there would need to be traffic control. With the bullshit cargo scanners, that would make smuggling much more difficult, but since there really wouldn't be cargo scanners, smugglers could rely on hidden compartments or falsified manifests and take their chances.
Basically, think
Babylon 5 - they got it right.
