Guess I already responded in the sad thread, but to move things over here, in my experience short term care of a healthy softshell involves a bucket. I've seen adult ones survive in a 5 gallon with nothing much but something to eat thrown in and something to rest on (usually a fallen branch) for over a week. Injured... if you've got a large plastic box -- the kind you'd normally use to store christmas decorations or whatever in -- and a smaller one, you could probably stick the smaller one top-down in the larger one, fill up to the small one's edge with water, then put the turtle on the small box and mostly call it a day. Find out whatever it eats, make sure the area it's in doesn't get cold, and you'll probably have done about all you can do. Maybe stick in some kind of ramp material so it can easily get on/off the smaller box. That sort of construction. Would be the best guess I can offer when the last time I had a turtle was fifteen or twenty years ago, heh, and none of the softshells were kept for any particular length of time (usually they were kept as pseudo-pets for a few days and then released or eaten, depending on the size).
E: Though if it's one of the smaller ones, you might just need the small box and a rock or somethin' to rest on. Maybe two, one completely out of the water and one partially submerged. Whatever's appropriate for the size, basically. Short term care wise turtles don't need much space at all, so far as I'm aware, just food, water, and warmth.