OK to get the sata connections outside the PS3, so you can use a 2.5 and a 3.5 sata drive , you will need something like one from here: http://www.tinygreensheep.com/index.php?main_page=product_info&cPath=11_38&products_id=986
Then if you have one. Try this.
1. Find the end of your new lead,the male end which plugs inside the PS3 drive tray.
2. Now leaving space to cover the cut wires, for insulation afterwords,cut off the Red,Yellow,and both Blacks about 30mm from the male connector end, leaving fore longer fly leads coming from the female end.
3. Now hunt the house for old Digi boxes that had hard drives in, like a DBox or some vox boxes that had a hard in, old sky boxes with hd, DvD recorders with hd, even old xbox1's had hard drive power that used a molex power connector that had RED YELLOW and both BLACKS. Some media drive caddy's had 12 and 5 volts coming out the transformer end because the drives were IDE and it helped to keep the heat down inside the housing, so you may find one there.
4. Now with your power supply, test it powers up ok without the parts you have disregarded in a safe place so you don't blow ya self up. and test its outputs.
5. After removing the power from the wall, connect the fore wires up like for like, Red-red Yellow-yellow but noting that in some cases the blacks are not the same ground, so just put the black that was next to the red,next to the red, and the black that was next to the yellow, next to the yellows, just like it was before you cut it.
5a. Staggered Soldered lay on joints would be best, but not always easy if you don't have the means to do it, so I would use small 12v choc blocks thats could be found in old toys that had the larger battery's, old low lighting units have them, and some strip lights, even inside old garden electrical tools have them.
6. After terminating, go over and make shore you have done a safe job, and there is no way of shorting out anything.
7. Now fit with the fixing/holding caddy screw PS3 end a peace of wire the same size as the red/yellow/blacks are. so you will have a wire coming from the drive try of your ps3, that will hang outside, Black or green, its up to you, its just to stop power spikes between the ps3 and your home made power supply.
8. connect it all up, not forgetting to join that fly lead to the drive with a fixing screw. then power the drive 1st, and let it spin up, then the ps3, never the other way round. Thats it.