You're right, if you can keep her inside, that will be safer.
However, I have had cats that absolutely HAD to go outside and became almost dangerous if I didn't let them out, so you have my sympathy.
Have you tried a leash? You would have to walk your cat, but better that than an injury. I find that my outdoor cat actually prefers to walk near me, so if he sees me outside he comes right over. I like to think it's love, though it's probably safety.
I think the easiest thing to build would be a kind of ramp. If you live near water, you might have seen the kind of thing I'm thinking of used to connect docks to the mainland. You would want a fairly wide piece of wood, or maybe 2, and enough pieces to reach from the window to the ground at a reasonable angle. Then you want some slats, maybe 1" wide and 1/2" high, like lathing or even from an old window blind. Nail or tack them to the board every 6" or so to provide steps or a grip so your kitty doesn't slide and can climb back up. If it's possible, you might try putting up a screen house of some kind and run the ramp right into that, so she's contained while in the great outdoors. Though that might involve enclosing the ramp in screen too . . .
Problems: It will weather, other animals might use it, the bottom of it will probably be some distance from the house thus taking up yard space, it might not be safe in storms, how will you attach it to the window sill or porch, and if you don't own the house, the landlord might complain.
I think the leash or supervised playing is your best bet. But that's just my opinion . . .