Had those. A decent design. But since I do not need 20mm clear aperture, I could use lighter / smaller. And would be willing to pay for this, but nothing is available.
With prisms smaller than 20mm, you start to run into some serious limitations as to what scope you can use the binoviewer on, if you don't want to barlow it heavily from the start, and with what eyepieces. Due to the limits set by the distances between the human eyes, you also gain less in size and weight reduction than you probably think, because you still need a stiff structure to ensure stable collimation and ability to hold the eyepieces firmly. If you want to go as small as possible, you also limit yourself to small and lightweight eyepieces, which means simpler designs.
This all means that such a binoviewer is unattractive to the vast majority of the astro binoviewer market, and for good reasons, which again means that no one wants to make one. However, in the past, several high quality microscope binoviewer heads were made, that seem to fit your needs. I've certainly seen some from Zeiss and others (forgot who) that were quite small and compact, though I suspect they were solidly made and thus still quite heavy. They were all of the "sliding eyepiece" type, instead of the hinged type that is now common.