I see a lot of unhelpful comments so far, so just let me say that I get where you're coming from and am also seeking a good option.
In informal situations, I use "cousin" or "neighbor"; at work with patrons I use "sir" or "miss" if I think I can guess the gender and "my friend" if I'm unsure. ("Miss" instead of "ma'am" or "madam" because apparently Easterners have a weird age connotation with those terms that I honestly don't fully understand.) I also use "friends" for a mixed group. But I've got nothing for a formal situation such as addressing a stranger on the street.
For those telling OP –and by extension, my Midwestern self– that just saying "excuse me" or "thank you" is fine... respectfully, no. That doesn't address the person. I get that manners vary by region, but it doesn't help us to be polite in an ungendered way in the regions we are.