We have assembled a potty-on-the-go kit, which includes a small folding stool, a pack of sanitizing wipes, some emergency pull-ups, and this potty seat. After a lot of research, I picked the weePOD because it is soft but still easy to wipe off, fits all toilet shapes, has suction cups for stability, and is all one piece, which makes it much easier to clean than those potty seats with the rigid plastic piece underneath that functions more to collect pee than to keep the seat on the toilet. Also, the hole is big enough for big kids, but not so big that it feels like they are going to fall through. There is a slight rise in the back that gives a bit of support and the splash guard is small but effective.
And the verdict is... SUCCESS. We started out by trying out our system at a relative's house so it wouldn't be a catastrophe if things went south. No need to worry, though. Collin sat right up with minimal support, did his business, and then stood up on the stool and leaned on me when he was done. I was able to prop my knees on the stool so I didn't have to maintain a full squat or touch the bathroom floor.
Next test: public restrooms.
UPDATE TO THE UPDATE:
Wednesday, Collin successfully used the potty at the PT center, Whole Foods, and the chiropractor's office. Everything went smoothly except for the time that my back pocket got caught on the front of his push chair and we both almost went sprawling on the bathroom floor. Luckily, I recovered and Collin didn't laugh too hard.