>>14,16
If "replace" means that the new thing is also going to be attached to your hand, then the most practical answer is a vagina or asshole, and masturbation would only improve. (You could even still hold stuff)
OTOH, you could ask for a girl, then get it amputated, since if it has its own body, it could survive quite nicely away from you. Detatching it from yourself would leave the door open for it running away, though.