Most any good adult diaper will retain the results from a Fleet or other small volume enemas but if you intend to walk around after the enema works incontinent pants over the diaper is a wise idea.
OTOH, if you're looking for a diaper that will fully absorb a full dose, adult large volume enema of 1500ml or more, pretty much forget it. While some diapers claim they can absorb that much liquid, that's pee and and comes out in a small stream over a minute or more and that flow is absorbed and sometimes turned to gel.
When a full bag enema works, you often expel a liter or more of water and poo within a very few seconds. AFAIK, no diaper can absorb that much, that fast. It will easily overflow the diaper and hopefully be contained by the incontinent panties and, if you are lucky, the diaper will re-absorb expelled enema solution sloshing around in the panties, but don't bet on it.
If you want to play with being forced to poo into a diaper, try a combination of laxatives as I've written before, or a small volume glycerin enema, given up high with a colon tube. Either of those will give you strong cramps and an uncontrollable, diaper-filling poo.