On some portables with a "free wheeling" knob, a small washer on the spindle adds enough resistance to "slow" the movement down..
Connect Systems CS750 and CS800, Hytera PD-365, Motorola DP4600
Raspberry Pi 2 and DV4Mini HotSpot
AOR AR-DV1 Digital Voice Receiver / eSPYonARD
WS1088 / TRX-1 / Whistler Q / UBCD3600XLT