For what it's worth, I'm not a fan of using bag enemas with diapers because of the mess and leakage. Very few, if any, diapers can hold 1500ml or so of water, even when covered with an incontinent panty,
If you don't have enough playtime for a powerful laxative to force a poo, suppositories or a small volume enema do the job. Glycerin suppositories are not laxative enough to make someone determined not to poo in their diaper go. After an hour, the recipient will be pretty uncomfortable but don't ensure a full diaper. For more certain results, a Dulcolax suppository, given u high with an applicator, will usually ensure a full diaper within an hour.
Another option is a Fleet enema, especially if given in the knee-chest position. They take about 5 minutes to work strongly, so the bottle can be given and the person diapered to await the inevitable. A Fleet, or preferably a Fleet Extra, has enough solution to make sure the person poos, but not so much as to flood the diaper.
My personal preference is a small volume, about 60ml, glycerin enema, given high with a colon tube, about 24 Fr, and a piston syringe. The well lubed tube is fully inserted in the knee-chest position and the person diapered and made comfortable, then the syringe used to inject the glycerine. Intense cramping will start within minutes and the diaper filled within a half hour. IMO, glycerin is the least messy way to make someone poo in their diaer.