Huh so futaba and JR use the same size boards...
A 50x33mm board is definitely doable with through hole 328, I can start working on it if you still want one. We can split the cost of ~£20 for 10 boards.
Which pro mini do you have? If you have the 5V version you will need to use a 3.3V reg for the RF22, and some resistors to divide the arduino SPI output to 3.3V. It could be quite simple if you have the 3.3V version, but I am not sure if my code will work at 8MHz.