Is the squeak constant if the pedal is held to the floor, or just during the transition in pushing the pedal to the floor?
Some possibilities, depending on the answer to the above:
First, if its just as the pedal is moving:
1. Is there grease on the tip of the clutch slave rod / input to the clutch fork? This may not have been done during slave replacement. Make sure the specified high temp urea grease is used.
2. It could be the clutch fork pivot needs grease (same type urea grease). This can be done with a little effort when the clutch slave is out.
3. You said its coming from the engine bay, so its obviously not the clutch pedal assembly inside the car.
Now, to the much more painful possibilities ( because they require removal of the transmission) if the noise is there the whole time the pedal is to the floor
4. Clutch release bearing (aka throwout bearing)
5. Pilot bearing
That's my list. Others may have additional ideas.