Hi
I’ve done this for an older PCI-X G5. But it was angle-grinder time 😉
The Mac has an eight-port Sonnet Tempo 8 eSATA card in it, and the external box has ordinary PCI-slot-cover SATA external port-replicator sockets chopped down to fit in the SCSI openings in the back of the box.
Inside the box there is a short SATA cable that goes to each drive from the socket, and an SATA>eSATA 1 metre cable connects the socket on the back to the Sonnet card.
The 8-channel cable bunch is a sight to behold, but the drive box is mounted directly under the computer, and the cables bunched together and tied back.
Inside the drive box I fitted a couple of 6-drive Intel Server hot-swap caddy SATA drive cages, which gives drive swapability without cable fiddling.
Also lovely twinkly green activity LEDs on the front panel – just like a proper drive bay 😉
As an aside its far easier converting a SCSI box to FW800, as you can get FW800 bridge boards that exactly fit in the SCSI openings.