Why under the costume? Halloween is the only night where you can get away with wearing nothing but a diaper, as long as you pair it with a cheap fancy dress shop baby outfit. I remember being out in a bar on Halloween and a very extroverted bloke wearing nothing but a Tena slip was running around the place yelling 'goo-goo-gah-gah', and nobody batted an eyelid! I doubt very much he was ABDL, but then how would anyone know?
In fact, my stock excuse for if my stash is ever discovered is to say I was planning to dress as a baby for Halloween and accidentally ordered far too many off Amazon, and now don't know what to do with them all.
They say Halloween is the one night of the year the dead can walk the Earth. It's not the dead, it's us!