I've read that some boat manufactures used to install a deivice in the fuel vent that would whistle from air escaping the fuel vent when filling the tank on a boat. I've searched high and low on the internet for such a device and have come up empty. The overflow containers made to stick on over the hull vent won't work on my hull configuration. I'm generally pretty careful about backing off on the fill rate when I think its getting close, but if there's a Harley or Peterbuilt pulled into the station it's pretty hard to hear the gurgle sound.
If such a device isn't available I bet a resourceful individual could retire early by coming up with one! It seems like such a simple concept it's hard to figure why it's not a standard feature.