The inside dimensions are 158.75 mm x 44.45 mm, which is the standard size of an IO shield.
I wanted to keep the PCIe slots and do as little cutting as possible. I created this snap on model to fill the gap between the case and motherboard port.

***I HAVE NOT TESTED YET***

I don't have my own printer, so I'm waiting on a friend to print it for me.
I'm doing a Mac Pro 3,1 mATX conversion.