You can try and make a tack bra, there are pretty good DIY instructions online! And would make you properly suffer if you'd have to keep it in for that long.
(e.g.
https://imgur.com/gallery/iWu6RZb/comment/1430896773)
You can have a large household object inserted, if you'd move you would feel it push in properly. Can be something from your bathroom / room like a makeup bottle, or if you're really looking for a challenge a deodorant bottle. If you're really brave you could do double penetration and also slide a marker up your butt.
I liked the suggestions about pegs on your pussy and bodywriting too ;-)