Actually, I built my own for my kitten...my camera's not working...but I used a 15ish by 15ish foot peice for the base.
I then attatched a 4 x 4 to that and put another level on it. I screwed on two 2 x 4's and attached another level. There is a ramp coming down from there. Then, theres a roof with a hole in it, so he can get up there.
I used screws for everything, so its more secure, and can be changed.
You can use PVC tubes and attatch to a wood base and wrap sisal around the PVC. We have a floor to ceiling tree that is all PVC with 3 wood ledges. We got carpet scraps for free to cover the ledges and used sisal to wrap the PVC.
Edit: Wow looking at this picture, the carpet matches to close to oru actual carpet it almost looks like there is no base! There is a base it just seems to be blending in.
I don't know what kind of tools you have available to use, but I use an air stapler to attach the carpet and screw the pieces together. A 4x4 works well for the post, but you can also screw two pieces of 2x4 together if that's what you happen to have. If you have one close by, go to a carpet dealer and tell them you're looking for remnants to make a cat post. They might give you a better deal that way. And don't be tempted to use really cheap carpet cause it will fall apart faster and make a bigger mess when the threads pull out. I've used sisal too, but usually hot glued it to the post which made it kind of a one shot deal. If you do use sisal, use some gloves so it doesn't chew up your hands when you wrap it. Let us know how it goes!
I will soon be starting to build a cat tower for my 3 cats as well. Something to take note of though......in doing my research to find enough sisal rope (I needed over 300 ft.) I discovered that the sisal rope that is generally sold in stores (home depot, lowes, local hardware stores) is oiled and has pesticides on it. A no - no for our kitties IMO. I got this info direcetly from a rope manufacturer. If you decide to put sisal on your cat furniture, make sure you get un-oiled sisal.
I ordered my sisal rope from birdalog or something like that. 100% natural rope and no oils. kittylova, I guess you were serious about building a tree..... sorry but I can't do it for you.