Hey JustShady, I have an M3903 too and I'm thinking about buying the CS50 without the lifter, because there is no way I can imagine to jerry-rig it to fit over the speaker, and plus I like my M3903 very upright, so the lifter would drop the handset to the floor whenever it operates.
My plan is to plug the CS50 into the M3903 headset port. To answer calls, I see myself having to press two buttons: the HEADSET button on the M3903, and then the answer button on the CS50 headset. That would be OK with me.
I'm just not 100% sure that it will work at all... What do you think?