Does the bottom of the tree taper down into a solid piece? Maybe you can take it out of the foam, and poke it into a drainage hole of an upside down pot and hot glue it into place? Or would a pot full of dirt hold it securely? Not sure how long the base piece is.
It can be simple...remove the basket and use a Christmas tree stand instead. You could also wrap the entire basket with plastic sheeting and caulk the trunk opening with silicone.
