I am using the Spektrum DX2E radio as the basis for my 2.4 ghz transmitter handle, as are other people. One problem with this particular radio is the lack of useable throttle trim. Being designed for car use, the throttle trim works to adjust the idle position in the middle of the trigger's range. It has no effect at the ends of the throttle travel, where we would need it most. I think I have come up with a relatively simple solution. I have not actually done this yet, but I can see no reason why it would not work.
Since most people won't be using the steering channel for anything except maybe a slider release, and the steering has a functional trim and also an overall travel adjustment, simply use the steering channel for throttle. Both the steering and throttle potentiometers have 3 wires soldered to the main circuit board, so simply connect the throttle trigger pot where the steering pot goes, and vice versa. Now the trim and travel adjustments work with the throttle instead of the steering. Of course the throttle servo will need to be plugged into the steering channel on the receiver in order to work.
I plan to build another transmitter handle soon and incorporate this modification. If anybody beats me to it, tell us how it works (or why it won't!).