The button itself on the power button activator is held on with electronics tape, which does not have good adhesion.
Will need good tweasers, really sticky packing tape, and clean sisors.
Tear down the phone till you have decent access to the power button circit board. Using the tweasers carefully remove the power switch tape and the dome activator.
I'd reccomend wearing a mask etc as to not blow away that dome, it is a pain to find!
Cut an appropriately sized piece of tape (I used a 6mm,6mm square and trimmed after) and use the tweasers to move the dome to the tape, sticking the dome to the tape like a bowl on a table, Then carefully slide the tape in and attach it with the dome centered on the switch.
At this point I would test the feel of the switch's click, and if it felt good then pop in the battery and test that it does power on the droid, removing the battery and cycling it a few times. (all subjective) Then when happy trim up the excess tape and close it up.