Okay, so I went searching for something that would really stretch my asshole to the limits. After spending a couple of days on a weekend wearing my large dilator and sitting on the Triple Ripple, I took some short naps with the 10oz bottle up my ass. That stretches me and is pretty close to my physical limits, but it doesn't go RIGHT TO the limit. (I hope to spend a whole night with the bottle up my ass pretty soon.) So I went shopping. I picked up a green plastic tumbler - the diameter at the bottom is equal to the 10oz bottle, and then the tumbler widens as you go up - or in this case as it goes up my ass. But I haven't tried it yet, I need to sand down the rather sharp bottom edge of the cup.
I also picked up a little dry snacks two-compartment container that is as wide as my 10oz bottle and then gets progressively wider in gradual ripples. It doesn't have any sharp edges (except at the rim, the part AWAY from my asshole) so...I gave it a shot. I managed to get it about 2/3 of the way up my ass. My butthole was stretched so wide, I could barely walk to get the camera setup.
Also, I am planning a hiking trip in a couple of months. I'm not going to say where, but other reports on the web say it's a great place to be able to get out and hike in the nude and not see another person all day, while still being able to get a cellphone signal in case of emergency. To that end, I have purchased a comfortable soft-frame pack and am getting used to wearing one of my oldest large butt-toys, the clear acrylic buttplug. At some point, I will be hiking the small mountains absolutely naked except for boots, buttplug, and backpack, and hope to get some self-pics at the time. In the meantime, here's a "dress rehearsal".