I've had the best results using a tub full of liquid H2O2 allowing complete submersion of the items being lightened, I think it's the easiest way to make sure you don't get any unevenness or streaking. Lining the bottom of the tub with aluminum foil (shinier side up) also helps bounce the light around.
Finding a suitable tub can be an issue, I recall using a cardboard box lined with a trash bag so I could submerge an A1200 top and keyboard (yes, I didn't bother pulling the keycaps off, but removed the metal and the membrane first.)