The vertical supports can be attached to the base with screws, nails, or wood glue. When you chose the PVC pipe to be the vertical supports, how to connect the hollowness PVC pipe to the wood base ? there are some solutions maybe you could refer to :
1. Cat tree connecter (column-connected units)
Pretty easy way to connect, but , the parts is not available, Some cat tree supplier may provides these stuff , only find available on taobao.com(an china website,just like amazon: http://en.wikipedia.org/wiki/Taobao ) .You could buy from here: Cat tree connecter from taobao.com . The seller provides custom made and wholesale .Sorry , I can not find it in amazon .
2. Create an wood connecter
This will need some skills, here:
There are some parts from amazon maybe you will need.