I recommend this plug or one of its smaller brothers in the SPD series:
http://siliconenozzles.com/SPD30-3-Inch-Diameter-Super-Retention-Silicone-Nozzle-SPD30.htm
Get it in the softest silicone, 00-20 Extra Soft. It's so squishy soft that it can't cause discomfort. My 3-inch diameter plug in this silicone grade goes in as easily as a firm 2.5 inch plug for me, and then it pops back out inside me to fill my rectum and apply very pleasantly stimulating outward force in all directions from inside my rectum. The plug's flat bottom keeps it in until I want it to come out. The flat bottom looks like removal would hurt, but it feels comfortable in this softest of all silicones. This plug is EXTREMELY COMFORTABLE!
I can almost always keep this plug inside me continuously for 48 hours if I follow a few simple steps. First, I do an enema series for a good colon clean out, (four 3-4 quart enemas and a long, thorough expulsion including waiting an hour for everything to come out.) Then I generously lube with olive oil my anus, all around my anus, and all over the plug. Water based lube can't go 48 hours without drying out. I pop in the plug, and I'm good to go.
The only barrier to 48 hour plugging now is gas. The bore hole of my plug solves that issue. I plug the bore hole with a 3-inch length of wooden dowel with a thumb tack in one end. If I feel bloated, crampy, or gassy, I can remove the dowel, release my gas, and replace the dowel. There's no need to unplug, fart, and replug.
As I sit here writing this in the wee hours of Monday, I'm in my 44th hour of being continuously plugged. It has not come out of me for a second. Far from my plug being uncomfortable, it is sexually stimulating right now as I sit here doing rhythmic contractions pulling my plug an inch deeper into me and letting it back out. I started Saturday morning after I did my enema series to clean out. I lubed up, plugged up, wiped away excess oil, and put on my tanga bikini swim suit, (50% butt coverage), to get ready for the beach. Pam and I walked up and down the beach for hours. The plug does not show under my tanga bikini. It's flange is very thin and very flexible. Saturday afternoon we worked out at our gym for an hour. I slept plugged Saturday night. Sunday morning, Pam and I went on a 30-mile bike ride. I loved those bumpy parts. Then we went back to the beach to enjoy the Sunday night drum circle. I'm about to go to sleep plugged for the second night in a row. When I wake up in the morning, I might do another enema series and start another 48 hours of being continuously plugged. I think I've got this long term plugging thing down now.
I've pluged myself over 100 times now for 48 continuous hours without any physical damage or injury. I just had a colonoscopy, and my doctor's report described my anus and rectum as having "no abnormalities". He even took pictures of the inside of my rectum--all pretty and pink.
I agree with Buttwoman's baggie precaution. Sometimes any plug, no matter how perfect, might be rejected by the mysterious workings of our bodies.