Sure, use a square instead of a triangle. That’s a good idea. Actually, we should use marks that are easily recognized by machine vision so they can be efficiently sorted.

Would it surprise you to hear that the plastics industry came up with that symbol in order to make it confusing to the layperson to differentiate between recyclables and single-use plastics?

"Source" (not a primary source): https://www.youtube.com/watch?v=PJnJ8mK3Q3g <-- which has some links to further reading/viewing and a few sources.