The PiggyBackTrim is a small module that fastens to your Pro Throttle via a Velcro strap and provides you with three axes wheels.  The big advantage is your throttle is not altered in any way.  You simply strap it into place, plug in the USB cable and you're ready to go.

The PiggyBackTrim is made from three wheel pots (from a Fender guitar).  The electronics are taken from a Saitek Cyborg Graphite joystick and shows up in Windows that way.  The wheel closest to edge of the throttle is the X axis, the middle wheel is the Y axis and the wheel closest to the button is the Z axis.  Most games will recognize this controller and permit you to assign game functions to the axes.  The wheels are surprisingly easy and comfortable to reach with your little finger.  The movement is very smooth and precise; ideal for use as elevator, aileron and rudder trim.