StrayerJ has added a buffer wagon and I have approved the pull request on git-hub, I haven't yet done a colour remap to differentiate it from the storage wagon or a save migration script to unlock the wagon on saves where the tech is already researched so I haven't done a release on the mod portal here. Feel free however to try it out and see if it does what you need. (See the "Source" link on the "Information" tab.)
If you are StrayerJ on git-hub then thank-you for the submission, I'll try to get a proper release out Soon(tm).