The issue with using other people’s machines is very occupation-dependent. For some people, it is a constant issue, and for others it almost never comes up. Over the years, I have run into it far less than I anticipated. If you have access to a USB port on those machines, it’s much easier, given that you can just carry a QMK board or a converter like a Hasu controller. Having said that, carrying a board is obviously only easy for those that prefer 60% or less in the first place. No one is overly keen to carry a full-size around. If you don’t have access to a USB port, then I think PKL is probably the best option (via your email or cloud storage). My general attitude is that if I’m not going to be on the machine long enough to warrant downloading PKL, it probably doesn’t matter if it has my layout in the first place. For such brief interactions, looking at the keys is not a big sacrifice.
When I’m on my own board, I try very hard to make sure that the keys are not labeled in a way that tempts me to look at them. I prefer blanks or unrelated symbols (I went with the Hiragana for the Planck). But actually I find that QWERTY (or QWERTZ) is nearly as good. The key is just not to have the keys remotely resemble the actual layout, so that there is no temptation to rely upon them, and instead just consult the layout in my head.
The corollary to that is that it’s much easier to remember your own custom layout. When you put a lot of effort into making decisons about every key, it really sticks with you—it becomes an extension of you, a physical manifestation of all the quirks of your personality. The keys are exactly where you would guess you’d put them.
The keycap mismatch issue is another reason that I’m a fan of small boards. A lot of touch typists don’t touch type all keystrokes. They touch type keys that are easily within reach, but then reference the keyboard when they need far away symbols, numbers, Fkeys. On a small board, there is no temptation or need to do that, because there simply aren’t any distant keys that require a hand movement. So I think going small tends to promote true touch typing (without any “cheat glances,” so to speak).