I made this a while back and sent it to SD. She's received it, so now I can post it:

I tried a variant with the dots coloured, but I think I prefer this one.

SD writes, "I tested it out on my family when they were here for Alexander's birthday and most of us saw it pretty quickly, except my mom, but she doesn't count because she is perpetually in a rush and not really noticing what she is doing."

So maybe it's not entirely squint free. I'll have to practise some more.