I like gearboxes but a standard transmission may come in handy, so here's a 1x12 Sram XX1 Eagle.
This sort of part is easy with a bit of planning, as long as boolean operations cooperate...
No front derailleur but a chain guide instead.
The derailleur, aka "the most enthralling thing I ever modelled".
I didn't do all the little variations on the cogs' teeth as my life expectancy is only about 80 years (well, with the real thing in hand I would probably have done it...).
The whole transmission, installed here on an invisible bike. Don't you dare steal it though, it currently has no saddle.