They have this fort building kit at the library. I have been wanting to get one for the boys, but every one I've found is super expensive ($60 for a basic kit). They build forts for free using my kitchen table and chairs, so I won't spend that. But I'd like to build them a kit, maybe with PVC pipe and connectors. That's what it looks like these are, just smaller, and I've never seen a 6-way connector in the plumbing section.
PS they built this helicopter today. They did a great job!