16:9 Guides for Square Comp?

[entry-newspaper]

Morning!

I'm working in an oversized square comp - for purposes of greater latitude downstream in post.
However, I want to have a 16:9 view-port guide in Cinema. What would be the best way to accomplish this?
The Title / Action Safe lines won't help, because those are only a percentage of the comp, so they're remain square.
The RS Camera does have a number of optional guides, but none of them allow me to precisely show the 16:9 I desire.
I'm sure I'm just missing a native tool for this, but could someone please point me in the right direction?

Thanks so much!