Make one?
A old toaster oven for the 'box', and a light bulb on a dimmer switch to regulate the temperature.
here's a better idea, let the kid make one.
BTW, you have a great idea - a box, any box, line it with foil, place the sample inside, cover with plastic wrap - and warm with an electric (one of those illegal incandescent) lights.
Add an inexpensive digital thermometer to record the temperature.
Cost -minimal
Kid safe, a 7th grader should be able to construct that.
I was always amazed at the complexity of some of the "science projects" built by the mothers and fathers of the students. Wonder what the kids learned from those?
If it's really the kids idea, their research, minimal parental help is all that is required.