Securing communication between cloud VMs without private networks. Any strategy besides building own VPN? Spiped?
I am going to suggest an approach that I would not necessarily recommend, because I do not consider it a manageable, scalable solution:

SSH port forwarding.

SSH is a Swiss Army knife in terms of its features. Virtually any port can be securely forwarded to any other port on a remote machine, and indeed through a remote machine to a target machine beyond.  Documentation abounds, but I really like the drawings in this answer. I use ssh across bastion hosts regularly in a manner that mirrors the diagrams at the link.

As I said, I don't think this is a good solution, but it might serve to give you some ideas and it could be one component of a larger solution.

Good luck!