Shared clipboard between multiple X11 servers (Xephyr, Xpra etc.)

I plan to run potentially dangerous applications *(browser, etc.)* on a separate X-server, but since each server has its own clipboard, it is impossible to copy the link / text from one window to another. Most articles suggest doing this using scripts using xclip and other similar utilities. But how to **CORRECTLY** create a common clipboard so as not to accidentally create a new vulnerability?