I've never used one, but I don't think it is dependent on the snake being too thick to fit through after eating. If that were the case it wouldn't work at all for young snakes.
The ones I have seen described were 2 liter bottles with the top few inches cut off and turned around and insterted into the hole, so the snake could get in but then wouldn;t be able to find the hole back out, which would be suspended somewhat into the middle of the bottle.