I've actually started working on this (original thread http://psx-scene.com/forums/showthread.php?t=40258 (Need a broken Network Adapter for custom HDD enclosure))
+Hollow out a PS2 network adapter install a 2.5" hard drive in it.
+Add a main power switch (like the old PS2s)
+Ensure the slim PS2 will still work on a vertical stand
-Install the DMS4 Pro
-Install LEDs for activity lights in the network adapter (where the RJ45 and RJ22 jacks are).
I installed the power switch by cutting the + trace coming off the center pin of the power jack and wired accordingly. Then I removed the modem card, and cut the RJ22 hole out to fit the switch. Works great, and now it doesn't have to be in perma-standby. Does anyone even use the modem? The switch was pulled from a broken Sony external burner, but I'm sure one from an ATX power supply would be the same size.
The 2.5 drive won't completely fit inside the network adapter, so I cut out a hole in the bottom of the network adapter. With a dremel, I got the edge of the 2.5 even with the side. I made another plastic piece to cover the hole (not yet pictured).
I finally got pictures posted today (01/03/06). hddoverview.jpg gives you an overall idea of what I'm doing. The 2.5" drive is below the Network Adapter leaving a very tight fit.
hddback.jpg shows the power switch I installed, and where the network adapter with hdd will sit. You can also see the jb-weld-married female IDE and ATX ends in the background. That will go inside the network adapter, and the male ends will go inside the PS2..
naback.jpg is a bad picture, but it's all I've got. It shows the silver plate on the back of the network adapter with the hole cut for the female IDE and ATX ends.
I hope it makes sense now.