Nice idea, but I agree, swiss chard is a big plant and those are very tightly packed. I would thin them out.
When I saw your title, I was thinking an actual (installed) window box:
https://f.tqn.com/y/flowers/1/S/v/F/-/-/ ... Kramer.jpg
I eventually want to get window boxes for my new house (once we can quit spending money for a while and catch up!).
I think rainbow chard would be beautiful in a window box!