Im a bit dubious of most Jell cap methods, thought I've heard succesful results with Lemsip flu caps.
Personally, the most reliable plugging method involves using a 1/2mL plastic tipped syringe, so one without a needle (you can usually find them at pharmacies). Pull off the plunger, Squirt a little bit of water based lube down the syringe, Crush the pill up and put it down the syringe, and then squirt down a little bit more lube. (Durex Play lube is really easy to squirt for this). Be careful, you don't want to use more than about 1mL in total lube, our you're gonna want to shit quite bad.
Then put the plunger back into the syringe SLOWLY. Force out as much air as you can. If you're really finicity, you can put a pin down where the needle would go and mix up the lube/MDMA.
The stick that syringe up your butt and squirt.
I'm also curious about methods using just just half a jell cap, but I haven't figured this one out yet.