I have learned now to wait until the last minute, grab an SS Conoy, then wait for convoy time to be over, but since i accepted the convoy before time ended, i have 30 minutes to run it. you get that overflow time to convoy, but plunder will be closed, so there is no risk of getting attacked, don't even need to worry about having support.

[EDIT:] this does work, someone in another group did this, and one of my group members could not plunder them.