If you want an effective enema that is small volume, there are options.
You can take a Fleet enema and just ignore the "2-5 minutes" that is on the label. Hold it until it spontaneously expels.
Another is a mix of glycerin.
Only a couple of times in my life I have done this.
Get a 32 oz bottle of either Gatorade or Powerade.
Drink the contents.
Wash the bottle out until you can no longer smell the drink.
Pour in 18 oz of Glycerin. Pour in the contents of ONE Fleet enema.
Add another 8 oz drinkable water This can be at room temperature.
Shake it up well and let it sit overnight.
Make SURE you have fasted at least 12 hours
Pour it into an open-top bag.
Take it in Simms position.
Retain as long as possible. anything past 7 minutes should produce a complete evacuation on one round.
Be ready. Have two clean washcloths ready, one as a bite rag and one in case you start to leak on the way to the toilet.
If you have fasted there is not a big chance, but there is still A CHANCE it could come out both ends at once, so make sure you have an empty plastic trash can or bucket in easy reach in case it comes out both ends at once.