Here's the one my wife and I put together a few months ago, about 2 weeks after we got Juniper.
The tube is 10" so it's actually too small, which we didn't realize at the time. Actually the whole thing is rather kitten-sized, rather than cat-sized, because we a) used wood remnants we had and b) scaled it for current Juniper, not future Juniper. Bixby loves the thing, though.
We found that it's very easy to use eye-hooks and dangle a cat toy of some sort from any platform, so there's more hanging off of the thing now. We're planning on building a bigger, 2-cat one next spring.