The choice of something to make a recipient fill a diaper with poo depends on the time available for play.
If you have 12 to 24 hours, nothing beats a full adult dose of Miralax and Senna. The Miralax will do nothing but soften the poo. And it really does the job The more you take, the softer the poo. I'd suggest taking two full adult doses with the suggested amount of water… one mid-evening, one at bedtime. The Senna pretty much only pushes poo out and usually starts to work in about 6 to 12 hours. Take 2x ExLax Maximum Strength at bed time, put on the diaper, and go to sleep When you feel the laxative starting to work… resist the urge until you lose it.
If you have about 6 hours, the Miralax again to soften you poo combined with the full adult dose of either Lactulose or Sobitol. You'll probably get a lot of gas in a couple of hours, followed soon by a very complete, impossible to hold, poo. If the urge isn't strong enough to force the poo out, wait a while The urge usually gets stronger with time. A full dose of Castor Oil will do about the same on the same time scale.
If you don't have more than an hour, oral meds aren't fast enough. You'll need a piston syringe of 60 ml to 150 ml, a F 24 catheter o colon tube, and some lube. For a solution, either one or two Magic Bullets Bisacodyl suppositories or 30 to 60 ml of liquid glycerin and enough warm water to fill the syringe. The recipient will preferably be in the knee-chest position. The catheter is well lubed, inserted inserted about 8 to 10 inches and all the solution injected and the recipient diapered swiftly. The diaper is usually filled in 10 to 30 minutes. Resistance is futile.