In my experience inflatable vibrating butt plugs make good milking tools though I’m not too sure why a properly inflated plug needs to be locked in place. If your gurl’s hole is getting a little slack then a few more pumps would normally solve the problem, generally plugs only need to be locked in place if they are not inflatable. Locking a plug in place is of course advised if there is any likelihood of cheating, such as when your gurl is left alone for a few hours carrying out chores.
me, oral-slut. :p