Actually, i use a riser found on Amazon.
The size is OK (the height), but it's misaligned with the fat back.
The Apple dual PCI riser is OK for that : the PCI connector is shifted.
I have a photo (from up) :
The first is too short and the card will not fit, because there is the hard drive cage.
The second is on the wrong direction.
The third is the same as the first.
Number for is OK in size, but PCI connector (male and female) are aligned, and it's not good.
And the last is too high (i can't use the Comm Slot riser). You can see the shift between male and female connector.
I search a riser with the shift (like the dual) and the right size (like the 4).