After my first session at the fortress (it was a great time - thanks Mistress Rey), I guess I can answer my own question somewhat.
One salt water enema (CVS brand I think) had a decent amount of water. I got three good 3 squirts out of it. You could also unscrew the cap and fill it with regular water. Then I tried one mineral oil enema, also a disposable CVS brand. It was about the same size as the salt water enema, but I think I only got 2 uses. It is safe to use one salt water enema bottle in a day and then use mineral oil as far as I can tell. The salt water enema said overuse could result in dehydration, so I tried to drink plenty of water. I did one or two smaller squirts of regular water a while after the mineral oil one. I think it might have taken 1 - 1.5 hrs altogether, but I took some breaks. It might take time (package said up to 15 min I think) for the water to come out, so you want to give yourself time for that to happen before the session.
If you fill the enema bottle with your own water, it would probably be good to filter it first. If you can't do that, you could run the cold water for a while before filling enema with cold water. Then you can warm it up before you use. Hot water will dissolve more stuff in it, so it could get more lead or other bad stuff from the pipes.
I would not recommend using disposable enemas in a public bathroom, but you probably could do it. The package says to lie on your side or do the face-down, ass-up position while you do the enema. But, you could probably do it standing up. It might not clean as much area, but that should be fine unless you're taking something huge in the butt.
The enema bottles were not that big - probably less than a foot tall and maybe 2" x 2" cross-section. You could buy one and save it; it shouldn't be too hard to hide if you need to. Just clean the tip off after each time. The cost for one was only a few dollars.
On the first enema, I had no drips come out of my butt while I was squirting the water in. The more enemas I did, the more stuff would drip out while I was applying the enema. There were only a few drops at the most, but you might as well do it over something you can clean easily.
As for the eating schedule, I ate very little food the day before and nothing the day of my session, which was in the afternoon. This was probably not necessary, but I had had a lot of food over Christmas, so I was not so hungry anyways.
Mistress Rey approved of my efforts, so I guess people could experiment with doing less than this and test if their asses are clean enough too.
PS - Sorry I forgot to give you a tip Mistress Rey. I assume that is the norm. Is it usually 18%? If I can come again, I will bring an extra tip for you.