SofEther VPN has an impressing set of features that stands out from OpenVPN.

This is one approach for what you need: Ad-hoc VPN

From Wikipedia:

SoftEther VPN is a free open-source, cross-platform, multi-protocol VPN solution developed as part of Daiyuu Nobori's master's thesis research at the University of Tsukuba. VPN protocols such as SSL VPN, L2TP/IPsec, OpenVPN and Microsoft Secure Socket Tunneling Protocol are provided in a single VPN server. It was released using the GPLv2 license on January 4, 2014.
The architecture of SoftEther VPN was designed for firewall penetration. Support for NAT traversal is provided, making it possible to set up a VPN server behind an organization's or government's firewall. Firewalls performing deep packet inspection are unable to detect SoftEther's VPN transport packets as a VPN tunnel because HTTPS is used to camouflage the connection.
Performance optimization was another objective for SoftEther VPN. It employs strategies such as full Ethernet frame utilization, reducing memory copy operations, parallel transmission, and clustering. Together, these reduce latency normally associated with VPN connections while increasing throughput.

Source code: https://github.com/SoftEtherVPN/SoftEtherVPN