For this project, you only need a large steel tub, a hole saw (or alternatives mentioned below), and some tape (I used masking, but electrical or duct would also work well).
This 17 gallon steel tub from Home Depot ($21.97) was the perfect size. The gallons aren’t particularly important, but you want to make sure the diameter of the top of the bucket (which becomes the bottom of the collar) is wider than the base of your tree stand. It also should be shorter than the bottom of the base to the bottom of your first branches. (You can also use the collar to hide a small platform under the tree base and make the tree a little taller!)
If the measurements are close, I recommend test fitting the base BEFORE you cut anything so you can return the tub if necessary.