Create a "knob" button using a Gesture Pad with the rotation gestures enabled. Here's an example of one live. This one uses a simple translucent button with a "divot" to indicate rotation, but you can use whatever image you'd like to make it look like any knob you can imagine. This one is controlling the volume on a Denon receiver, and the volume level shown kno the knob is actual live feedback from the receiver.
Knob buttons work great and look good, get your own image. Using one myself for volume control, finer control than I used to have with an iRule slider.