On his latest car (MINI Cooper Mark I), Sheepo (Fernando Benavides) used a mini sequential 4-speed gearbox, or wasn't he the master of sequential gearboxes.
After many requests Sheppo decided to publish some building instructions, to help you build your version and use it in your own MOCs.
Find all the details at Sheepo's blog or download the instructions here.
And pay attention to some remarks he makes near the end of the post in his blog, as you need to replace some parts, which does not exist in LDD.
After building this one, if you still want a challenge, maybe you should try the instructions for Sheepo's 7+R sequential gearbox...