Need a place for my 3 cats to sleep and play .
Here is also a video tutorial for building one: Design your cat tree. ... https://www.youtube.com/watch?v=nuDVi_4e__Q
Here is one made from repurposed drawers which I love! You could find drawers and build your own https://www.pinterest.com/pin/743868063430380773/
We used a pine tree to create 2 posts about 4 feet tall each attached to a base that was about 2 foot wide by maybe 4 foot long and put a 2x4 between the two slightly more to the top than bottom and built a box with holes on each end to attach to the 2x4 for a sleep area for ours. Covered the Pine Tree with sisal. Our base was made from a bullet proof window so we did not cover that but you could use wood as a base that could be covered with carpet.
The trick for your base is it needs to be large enough and heavy enough that it prevents your tree from toppling over when it has the additional weight and movement from your pet. You could make a box as your base that would allow you to add bricks inside for additional weight.
We have made straight scratching posts for our fur babies before that would topple when they would scratch on it because the base was too shallow and had no real weight.