For the hinge use 2 x M3 screws (any length between 15 and 30mm).
No need for heated inserts, just screw in the plastic.

This remix has an overall height is 25mm; holds a mouth guard max 22mm height.

Also adjusted to have a slightly tighter case tolerance and slightly larger hex cap socket hole.

(As pictured: PETG; 0.4mm nozzle; 0.15mm layers)