I made the mistake of buying a cat scratching post that was too small for my cat to use comfortably... So I gave it away (to someone with a kitten) and thought I'd make one for my cat, Dylan.
For materials, I used a pallet that I found in my recent "how to get wood for free" video. I'd use the stringers to make the post, and the slats to make the base. I dismantled and de-nailed the pallet using a crowbar.