Thanks to the exacting measurements and advice in this thread, I was able to replace my missing cover as it probably flew off on some highway and landed on an unlucky someone's windshield.
This was the amazon item I bought and I think the model number may help if it ever disappears. 96219 or D5000EBQK
a lot of listings will say it won't work on the G80 (or 2015 Genesis Sedan in my case) with the panoramic sunroof maybe because they know you have to replace the whole structure, but since we're going the route of gluing anyway, as long as the cover fits dimensionally, we're good. Thanks to
@joegr and
@ted33021, we have that. (3.02" wide at the back and 5.93" long. It is 2.62" tall at the back) I've tripledchecked and measured with my calipers too. I think
@joegr might have been referring to the base and
@ted33021 gives a closer match with his pics of the actual cover.
So, I feel more important in any listing is a pic with the dimensions like the one:
I'd say the same goes for ebay, but I got mine on amazon due to "easy returns" in case it didn't fit. Originally, I thought based on the measurements, it would sit over the entire structure, but when it arrived, it ended up resting on the lip pretty properly. It was just missing a bit of the vertical curve that follows the roofline. Also, I suspect these guys may have measured the lengthwise curve too instead of just the straight line of length to get their 6.2 inches.
Now, the thing won't rest on properly because there's inner screw posts that get in the way of the existing screw posts that broke off from the old cover. Enter our friendly rotary tool. I thought of grinding off the ones on the roof, but then I realized those are the ones holding the antenna assembly in place! Much better to grind off the posts in the cover as we're not going to screw into it anyway. I suppose if I had dropped the headliner and properly removed it from the inside, I could've maybe screwed it back into this one. I think the posts matched, but if we did that, it'll probably break off anyway after time so the glue is still better.
Despite this pic showing only the front 3 ground off, I did all of them.
There, now it rests better on the lip. As mentioned before, it doesn't match up perfectly vertically because I don't think they accounted for the roofline curve, but I'll live with it. I'm already more than happy that it's pretty much a proper match all around.
glue time. thanks to
@ctrcbob for providing a few glue alternatives. Found E6000 so I went with it.
So I just spread a bead along all the inner edges.
oh no.. I did too much.

The sun was in my eyes on this side
I just ran a paper towel to smooth it out. What a mess. Maybe a finger or a card would've worked better. Treat it like you're sealing your tub or something.
luckily, a little bit of alcohol on a paper towel cleans it up nicely.
All done. Maybe some may obsess over the imperfect glue bits in the seams. Feel free to substitute with an alternate glue and seal it better, but I'm good with it. As far as I'm concerned, the bead on the lip inside is holding everything pretty good and tight.