Keeping it in a box next to her and lifting it in and out isn't likely to work.
This is not my experience. It may take a while, but I’ve only ever failed to get a ewe to accept a lamb a very small number of times (out of hundreds.).
Personally, I don’t recommend the ‘getting tough’ option. In my view, if you can keep things calm and gentle, the ewe will eventually settle and accept the lamb. If you build an experience of unpleasant things happening to her whenever the lamb is brought in, that can result in more aggressive behaviour for longer.
Generally, even with a tricky one, after a few days, the lamb smells of the ewe and she will start to accept it. I have had it take a week a few times and once it took 10 days, but in most cases a few days is enough for it to be safe to leave the lamb in the pen with the ewe, and one day you’ll come in and find them curled up together.
If you can make a spot in the pen where the lamb can get away from the ewe, you can start to leave it in the pen with her once it’s strong enough. Keep within earshot at first, so you can intervene if she does start to get aggressive with it.
Some people recommend bringing a dog near to the pen, and the ewe switches into protecting mode. In my experience, it can make the ewe go all red mist and beat up anything she can reach, including the lamb, so if you are going to try this, do it with caution.
This possibly all sounds a bit as though it’s giving the ewe a traumatic time and make you wonder if you’ve done the right thing. But one day you’ll have that ewe and lamb out in the field and she’ll be absolutely devoted to that lamb, and then you’ll know you did the right thing.