ULANZI VF01 28mm Optical Waist-Level Viewfinder w Switchable 35/50 Framelines, 3-Group High-Clarity Lens, Magnetic Flip Hood, Universal Cold Shoe Mount, Retro Aluminum & Leather Design(Black)

Mirror? Triangle prism? No need for pentaprism for such a vertical angle. Or you want to name the whole viewfinder device? Ask yourself, what are you trying to achieve. It is clear, what you loose -- the actual perspective, framing of the shot and synchronicity of the focus. May be better look for a camera with a flipping LCD screen like Sony Alpha? Or a right angle viewfinder eyepiece? Just don't be seduced by cheaper models -- I have one and am not very happy with the convenience. Better look for live test before buying. Edit: found exactly the one you are looking for, a waist level viewfinder: https://www.amazon.com/ULANZI-Waist-Level-Viewfinder-Switchable-High-Clarity/dp/B0GCKCXSF3/ref=sr_1_5