@Naughty_Elf
If you're inclined to play this way, it can be a lot of fun… and hopelessly humiliating.
But, I'd suggest you don't simply want to take a massive dose of laxative that will make you poo soup for hours. Of course, if you like a diaper full of sloppy poo, go for it, but it's really not my thing.
My preference is for a much more controlled experience… a very large, soft, but not sloppy, poo… that starts with a full, packed feeling in the rectum, and a slow realization that one has a need to poo, but don't want to go in my diaper. Over the next half hour or so, the feelings grow in intensity, but the diapers cannot be removed. At some point, the urge to poo becomes desperate and crossing legs to hold it in becomes fruitless. Finally, comes the realization that resisting the laxatives one was given cannot be denied, and the diaper gets quickly filled with poo, the consistency of mashed potatoes. Oh, that warm fullness!
If you are going to play I's suggest a high fiber diet and/or supplement for a couple of days to be sure there's lots of poo waiting to come out. Then, six to eight hours before playtime a dose of Miralax plus a dose of Senna.
To get the dosages right, so you get desired combination of softening and urge, I'd suggest trial run, taking the fiber and meds and pooing on the toilet. That way, you can adjust the dosage to get the result you like, without a huge mess to cleanup.