A few years back, I made a window in an old WD200BB. Cool part was that as there was only one head on the bottom, I was able to draw patterns on the top surface with retroreflective tape, then with a bunch of ultra bright leds flashing a short pulse at 120 hz, the pattern appears stationary.
It was quite impressive in the dark! My tip would be to find two identical drives, one to play with and the other to leave untouched until you are ready to swap the new lid. Old WD's are quite nice because the lids are relatively thin and easier to cut without deforming the shape. Small windows are easier to make that changing the whole lid, the head support, gasket and breath hole are already done.
This year I did away with the lid and put the whole HDD in a bell jar! Video is on youtube:
http://www.youtube.com/watch?v=Q5un_GkwjCsGood luck with your hard drive.