Keep it simple.

I like the idea of riding around with a prune label on my bumper, but
someone with a new/restored Box may not...

The ascii graphic sounds good, but if we were to start to make it somehow
"politically correct" by including graphics of all Box types, then it gets
complicated.  Remember that it has to be something easily recognizable from
a modest distance, and at a glance - you might someday whiz past a Box
stuck in the breakdown lane at 65 mph.

Wolf