I agree, 18" will not provide sufficient pressure and will not result in a good enema.
I use a door hook and metal S hook to hang an enema bag on the door and I lay on the floor to give a decent pressure. It works very well for me.
However filling up the bag was a bit of a problem as I don't want any liquids spilled. So I have just got myself a nice stand, a bike repair stand which is perfect. It works better than I could ever of imagined. It folds out, is very sturdy and the bike clamp holds the S hook that I hang the bag on perfectly. It should prevent any mistakes such as the S hook getting knocked off the stand. I do use a large bucket beneath the enema bag to prevent any spillage while filling it.
The bike stand that I got a couple of weeks ago is absolutely perfect and I am very pleased with it. I can always use it for my bike if the need arises.