Millimeters is correct! If you need the Solidworks file for anything I am happy to share that as well.
I personally used a lab power supply just because I had it and it let me easily change the fan speed as needed. If you don't want to go that route you could get one of the cheapo boards from Amazon that lets to power it with the ATX PSU and set the fan speed that way. The motherboard headers *might* be able to do it but make sure the fan doesn't draw too much current if you try that. I have not looked into what the normal power delivery of those ports is.
I knew it was Solidworks, being a user myself I could recognize it due to the visuals being very distinct. I dont particluarly need the files though, the STL is sufficient for me but dont let me stop you if you wanna share. The fan mount on it's own is already more than enough for me personally.
You are right though that it would be too much for the headers on the board for that power draw so I was thinking what other options are there. Like are there any similarly shaped blower fans that could be used that demand less power? Like surely there's be a high enough airflow 2 blower design without going back to 40mm fans, even though I know that's silly it is an idea I would pursue.
For example, if a motherboard header can handle 1A of draw (and let's be real, less than that is usually maximum, so 0.75-1A), then it doesn't seem that bad of an idea to have 2 paired together fastened with a bolt and nut, powered off 2 different headers even if it would be a bit of a wiring mess.
Alternatively a solution where you can get molex power/sata power to cover it wouldn't be a bad idea either assuming you had enough connectors free. This definitely could be a good idea if it wasn't for the fact that's a lot to draw off sata power, especially the 12v portion, so 2 sata power wouldn't be the end of the world either.
Another solution is 6pin PCIEe power connector to power it but that seems silly.
As for the GPU itself, how have you all been powering it? My current setup is ready to be a housefilre since I'm yoloing with a 6pin PCIe to 8pin EPS especially since I have no idea what this thing is even drawing. The cable FEELS fine, but it hasnt even been under a load, it's just existed so if anyone has some ideas, I'm open to it. Second PSU wouldn't be bad either.