Flexbox item square aspect ratio

I am experimenting with flexbox. I am trying to create a simple layout with some colored rectangles (all in a single row). Ive got a basic flexbox layout working, but I would like to constrain the aspect ratio of the rectangles such that they are square. I cant seem to find a way to do this and most of the examples I find around the web are using plain css to accomplish this, not flexbox. Any ideas?

One thing I also noticed when comparing JUCE’s flexbox implementation with the css flexbox implementation is that JUCE is missing the space-evenly value for justify content.