Seeking window mounted (or DIY) ERV or HRV solution

[BillS111]

Hello - I live in a rental, so i am not allowed to do duct work or wall work.

Does anyone know of an HRV or ERV system that can be window mounted, like an a/c? Or is there a 'workaround' DIY solution where i could take one of the 'duct-work required' units and just sit it on a table, running ducts through a window?

My goal is to keep the Co2 ppm in the low 400s in a 300 sq ft apt, does anyone know the CFM rating that would be needed for this?

Thanks!