The absolute HARDEST ENEMA TO HOLD I have ever experienced was this.
Take a regular 32 oz bottle of Powerade. Drink the Powerade, and wash the bottle in warm water until the smell of the drink is completely gone.
let it set, with the cap off for a few hours to dry.
Pour 2 6-oz bottles of glycerin and the contents of ONE Fleet Enema in the Powerade bottle.
Add half a fleet bottle of water Shake well
Let it set overnight and don't eat any solid food or consume any alcohol for at least 3 hours before going to bed.
In the morning, before eating anything, or drinking anything but water, Soak that Poweade bottle in HOT water until it is warm but not hot to the touch.
Have your old washcloth ready and in easy reach to use as a bite rag or twist rag.
Have a clean dishpan within easy reach of the toilet, just in case it comes out both ends at the same time, because this one CAN!!
Pour it into an open-topped bag, get into Simms position, and take it as an enema just that way.
Like any enema, just try to hold it as long as you can!! Doubtful you will get to even 12, let alone 15 minutes. Have a second washcloth nearby that you can stuff in if you start to leak on the way to the toilet.
Everything WILL come out, there will be no need for a second round. With the level of nausea, I doubt you'll take another one anyway
It will take a couple of hours before you feel normal again.