Here is my conversion of a HobbyKing cheapo pistol grip mated to an old Fox handle. I wanted to use the Fox handle because it is a steel strap with all the necessary bends already made, plus the adjustable cable. It needed to be lengthened as the grip was too close to the front end to allow enough room to operate the trigger comfortably, so I cut the front end off, and added the 1/8 fiberglass PC board pieces (the green parts), which also saved the work of trying to cut a slot or clearance for the trigger. Then I made my own grip parts.
The radio mods included removing the centering mechanism from the throttle pot and mounting a spring internally to pull it to the full low speed position. Also removed the steering wheel, but not the steering pot. I made the wood fillers behind the trigger to give me a mounting surface. For power, the 8 AA alkalines were replaced with a 3S 1000 mah LiPo, in the box on the bottom. Did some testing to see how much transmitter time is available and it looks like about 10 hours on a fresh charge so would take about 75% of the capacity. So, plenty of time to fly as many events as needed with one transmitter.
Cost was pretty minimal - the transmitter/receiver was about $30 (including $9 shipping) from the HobbyKing USA warehouse. I have a couple of servos already but if bought with the radio, they would cost ~$5 each. The LiPo was about $9 if bought new, but by the calculations above, the 8 AA's would probably last all season. The Fox handle was still new in the bag, but I've had it for 25 years. The rest was just from various scrap bins.