After having started my own project to make a general purpose radio module that would work with multiple protocols for controlling RC models I found that to no surprise, someone has already done a lot of work in designing one: https://github.com/pascallanger/DIY-Multiprotocol-TX-Module
These are similar in construction to the one I had designed in that they use the ATmega 328P and a selection of RF modules commonly used by different manufacturers (in the case of my module I only got as far as using the A7105).
They are available as a complete kit for a fairly reasonable price at Banggood and fit very nicely into an empty JR module case from a FrSky DJT module which can be bought from T9HobbySport after cutting a section of the top panel out to be able to access the protocol selector and bind button.
For slightly less you can also buy a four in one RF module (identical to the one used in the fully assembled modules), also from Banggood.
I'm using this to build one of the modules into a Turnigy i6 that I'm not using much these days, more on this in a later post when I actually have this finished.