Does anyone know / have general ideas of which other manufacturer's rear diffusers are roughly the same size as ours?

I really love the look of visible tips like those on the IS350-F, and I want to build a rear valence to showcase a single perfectly round opening on each side. I looked at the other options out there (ready made) and they're either way too expensive, require quad tips, or is the GSF bumper. I truly want to make my own...

But, rather than start from scratch, does anyone have an idea of size that's roughly close? I was looking at Audi S7 and others, but haven't made any purchases yet. I know once I have the "right" part, I'll have a lot of work ahead of me, but my wife says I need a project anyway, so let's do this.

 

On further inspection, it might be better to pick up a "walker style" diffuser from eBay and just cut and mold the openings.