Smurf -- There's a button for that!
I came up dry in Google Images, but I've seen (in magazines) roll cages that have the door bars on hinges. So it's open door, open gate, get in, close gate, close door. OTOH there's this:
T-bone that!
I should go back and bump my Messerschmitt thread. There's no actual design there, but what I was talking around is a reverse trike with a monocoque tub and a tube frame roll bar above the beltline. The MaRussia supercar has a nice detail for the join:
Similar to the Porsche (962?):
...and since I broke my own self-imposed rule; Here's the door on the Nils;
Hope the O.P. doesn't come back and spank me.