The Choosy Express: the Yankee Boogle with a switch.

This product is a modified version of the Yankee Boogle to have select fire functionality without modifying the receiver or FCG. It is designed to function with Milspec pattern AR receivers and FCGs. The Choosy Express does not fit with lowered shelf lower receivers nor M16 FCGs. The files included have a few options for the selector that change the lever design.

Printing Instruction: Print the both pieces in its default orientation. This is with Choosy on its flat side and the selector with its axis at a 45 degrees and the lever horizontal. Recommended layer height is .2mm. The selector should be printed with support for angles greater than 45 degrees. The geometry on the selector's notches might require some cleanup or trimming to work properly. The Choosy should fit snuggly into the receiver with little to no force applied, but should not have any slop or play; some sanding may be required on the flat side.
Print with PLA+ at that filament recommended temperatures with 99+% infill and 1+mm wide walls. 

Install the selector as a simple replacement for the standard safety selector. 
Install the Choosy Express into the lower receiver with the rectangular notch towards the rear and facing upwards as one would with the Yankee Boogle or a DIAS. The notch on the selector should mate with protrusion on the Choosy Express.