Just throwing this out there, but instead of a concrete tube as the core, you could start with a 4" PVC pipe. You could cut the pipe into sections and then put the pieces back together with various types of PVC splitter fittings in between each section. Your branches would extend from those fittings. You could even bend some of the smaller pipes with a heat gun. It would be up to you which pieces you permanently glued together and which you just wedged together in order to pull apart later.