Guess what, I didn't see one at the Halloween store, a rectal thermometer costume.
Were I to have one, of course I'd have to make it. My thought is that it'd be very minimal, like a round board, a dowel?, painted to look like a thermometer, with a nubby on it. Then it'd be down my back, with the numbers and the red line and nubby facing out, over my head about 2 feet. Then for the giveaway coloring, melt a big Hershey's chocolate bar and smear it on my face and forehead and some on the thermometer itself.
What else there is to say about it, I can't think of much. There might be an easier way, a better way, and a funnier way. But that's my idea.
I ran the idea by someone I was talking to, and she didn't like the idea one single bit. But she was also a little ticked off about something, so that might've skewed her opinion. Her idea was that the rest of the thermometer would be painted on a white shirt, then the part going up my neck would suggest continuity. That's true enough. But even with that good suggestion, she couldn't get over her opinion that it was disgusting.
But it's really all theoretical anyway, because I don't have anywhere really to go on Halloween. Just stay home and give the kids who come by their little treats.