Feral cats won't use shelter

Hi all. First post here. My parents have about 5 cats in their backyard, including 2 younger kittens. It's getting quite cold in the winter in Canada, -15c, so we decided to build a little shelter. I got 2 plastic boxes, put the smaller one inside the bigger one, added some insulating foam around the smaller box, and made a hole for the cats to get in and out. My parents have been putting food and water outside during the holidays.

We set up a camera and saw one go inside and near it, but it's gotten very cold today and I'm a bit worried. Any tips to get them to be inside the box, instead of under the deck?