It looks like they used the original vent frame. Maybe they put stretchy film over the left opening, and filled the bubble with bondo to make a male mold. Then epoxied the back side to make the pocket that the gauges mount into?
Any advice on how to build such a thing? Alternatively, I'm also thinking about an insert that fits in the vent hole rather than modifying the frame. I'm trying to fit my LinkPro in there, and keep it looking professional.