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.
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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.