Usually when someone buys a cat tree for you it’s either because they think it’s a great gift or because the cats are destroying the furniture and curtains. Cats have an instinctive desire to exercise their claws and legs and they are either going to do it to a cat post or climbing tree or to your furniture - Which leaves marks… I suggest seeing if you can return the too small cat tree and get one that’s bigger or more stable. It would be less wasteful than just throwing it out and achieve the intended goal… Or if you are a handy you could get a 2 ft X 2 ft piece of 1/2 or 3/4 inch plywood and fasten it to the base of the cat tree you were given and cover the new base with some carpet or rug. Home Depot, Lowes, etc sell cut sheets of plywood pre-cut or they can cut it for you. The best way to fasten it to the original tree is via wood or long sheet metal screws up through the bottom of the plywood into the base of the original (pre-drill the holes in the plywood for the screws to go through). This would greatly improve the stability of the cat tree by extending the base and adding additional weight to the base. Alternatively depending on the construction of the original tree you might could use “L” brackets to hold the post to the new base. But the best is to leave the original base on and just fasten the new bigger base to the original.