I know that PortableApps already package the original RBTray. However, Benbuck's fork of RBTray offers two other ways (in addition to the original RBTray's right-click on the minimise button of a window) of minimising a window to the System Tray. The two additional ways are:
Hold the Shift key while Right-clicking on the program's title bar.
Pressing Windows-Alt-Down on the keyboard (all at the same time).
I have tested this with Thunderbird Portable v60.0 and find the addition of those two ways to be very useful -- depending on how the user has Thunderbird Portable configured. Therefore, I think PortableApps should package this fork of RBTray. I would think there shouldn't be any problem with this because the fork (like the original) is open source.
GitHub home page for this is: https://github.com/benbuck/rbtray