Another "thank you for asking this" because I'm curious too.
Ideally I'd just use my daughter's room, but I assume your supposed to close the door and we live in an old house with touchy doors and 80 years of paint--and there are two of them into her room, one from the hall and one from our bedroom. Anyway, I think you get the picture...she'd go in one and out the other before I'd get even one of them closed properly. So, I've been using her crib (only a couple times so far though) and share the fear of her getting a negative association from one of her favorite places. At this point though it seems like being in her crib has a calming influence on her when she's stressed out.
I've also tried this little vestibule just inside our front door after reading Burton White's "separation-from-their-favorite-person" or whatever that acronym is. He suggests a gate, you on one side toddler on the other, but again there's not really a good place in our house to do this. The vestibule has a door with glass window panes, but of course the door doesn't really close. For obvious reasons I wouldn't feel comfortable doing a full time-out there but I have used it more Burton White style--just have her there long enough to be upset about not being with me and then back out.
In the end, the only place I'd really feel comfortable if she was in a full tantrum right now is her crib. Not ideal though I think.