http://feralcatfocus.org/colony-managem ... m-fosters/
You have a big wire dog crate. Inside you put a water bowl and a litter box. You bring your cat home from the vet in a carrier. You put the closed carrier inside the dog crate in the position you want to leave it (for your cat to have a little house inside the crate). Now you take a yardstick or something similar and run it through the wires of the dog crate from side to side in a way that prevents the carrier door from opening. Now you are safe to unlatch the carrier door. Close the door of the wire crate, and use the yardstick to nudge the unlatched carrier door open. At this point I recommend finding some way to secure the carrier door in an open position, even if you just use a twist tie and fasten it to the wire crate or whatever. Now your cat can go in and out of the little house inside the wire crate. It is even cozier for her if you put a sheet over the wire crate so she feels safer.
Any time you want to feed her, or clean her litter, you can shoo her into her little house, use the yardstick to nudge the carrier door closed and then reach in and latch it. Do your cleaning, then open it again for her. This also works when you need to get her into her carrier for a checkup at the vet.
There is a trick for really frightened cats where you are trying to shoo them into the carrier and they don't want to go. Sometimes they just go on top of the carrier and do not want to go into it. In that case, pull the sheet back from the wire crate until she realizes she is "out in the open" and chances are she will then choose to go into her carrier and you can use the yardstick.
Since she is going to be spending a length of time probably hiding in the carrier and sleeping in it, I would choose the biggest one you can find that will work inside the wire crate.
My biggest worry with her is to be sure she is eating, otherwise she should be fine. Also, I would recommend putting her in the quietest room you have, such as a spare bedroom, where she will feel safer.
Which leg is it? Can you ask them if she is going to be wearing a collar? That will make a difference on how you do her housing, right? If she has a collar I don't know if she can get into the carrier.
Also, you might ask in advance whether it matters what kind of litter you use. I think something like Yesterday's News newspaper litter might be better than clumping or clay(?) but the vet can tell you.