If you look at the thread pattern on say the Cleanstream 10" Silicone Enema Nozzle,it appears almost identical to a bag plug aperture.. However some adaptor type nozzles are usually "Yorkshire" threaded as in universal pipe--Very close to, if not same as 3/4" York.
1) www.amazon.com/Cleanstream-Silicone-Comfort-Nozzle-Attachment/dp/B00767NASE/ref=pd_sim_hpc_2?ie=UTF8&refRID=08HBN4EASJJ5ZZWN1RXT
* Have a read of the reviews down the bottom of site page !!.. so who's been enjoying their Enemas then.. 😁 Bet ya chuckle !
2) www.amazon.com/Medical-Grade-Silicone-Comfortable-Painless-PurEnema/dp/B00AY4IFFE/ref=pd_sim_hpc_4?ie=UTF8&refRID=1XQ4D7CTW1X8TYAHE1DC
* Comfortable-Painless huh !!...I reckon there might be a few that could think otherwise.. 😈
The latter being readily adaptable to a quality vinyl water bag (1 gallon & upwards) from camping/fishing & disposal outlets, as the PVC ferrule can be fitted into the fill cap.
Solar heated shower bags another alternative that can be modified by applying a little ingenuity.
OPTIONAL :
A) Ensure you cut a decent piece of high density foam of correct thickness into an oval shape...the bum will appreciate it !!.
B) Fabricate a snug fit (hole saw does it perfectly) centre opening to secure over cap.
C) Purchase a simple inexpensive one way Flap Valve (small nylon disc with rubber flap attached) slip inside screw cap & you are ready to roll.