A others have said, the solution choice depends on the desired effect. My suggestion goes in fairly easily but soon causes strong cramps and becomes very hard to hold, even with a retention device. The longer it's held, the more intense the effects.
The solution is made with 1 to 1.5 Qts warm water, as warm as comfortable but not hot.
Dissolve two Magic Bullet suppositories in the water (water soluble Dulcolax, 10mg per suppository) and add 1 Oz of liquid Castile Soap, like Dr. Bonners.
Give the solution as quickly as comfort will permit with a well lubricated colon tube (#22 to #32) inserted at least 5" and preferable 8".
The bag should be held low, not elevated more than about 2 feet. The goal is to get the full bag of solution into the person before the laxative effect begins.,
often 5 minutes or even less
As soon as all the solution is all in, withdraw the colon tube, pressing a wad of tissue against the recipient's anus to prevent leaks, then promptly plug them, preferably with an inflatable plug. Then just wait for results.
Be prepared in case there is a messy accident. Plugs don't always stay in.