I started feeding a stray cat a few months ago. She is good with me but will not let anyone else pet her. I didn't realize she was pregnant until 3 weeks ago when she gave birth to 4 kittens. I have them in a plastic, low sided container in a quiet space in my den. She is a great mom but now the kittens are starting to attempt to climb over the sides. I need something that will keep them contained but allow the mom to go in and out. I don't want to spend a lot of money because I am not planning on keeping them. As soon as they are old enough, I will be adopting them out. Any suggestions?