Simple model for inserting a M5 Hex nut pocket in your designs. This was made oversized to accommodate for print shrinkage and glue "channels".
Flat to flat measurement is 8.6mm and 9.8mm corner to corner. Pocket depth is 4.5mm

Just a few pointers for the glue up process:
Never try to drop glue into the pocket and then push the nut in with your fingers. The glue will get everywhere. Instead, use the bolt as a handle.
Prep the Bolt: Rub a tiny bit of chapstick, wax, or oil onto the threads of your M5 bolt. Glue will not stick to grease.
Mount the Nut: Thread the M5 nut onto the very tip of the bolt.
Apply Glue: Put a tiny dab of super glue (CA glue) or epoxy onto the flat outer sides of the nut. Do not put glue in the plastic pocket itself.
Insert: Use the bolt like a screwdriver handle to push the nut perfectly straight into the 3D-printed pocket.
Unscrew: Hold the nut down flat for 10 seconds, then gently unscrew the bolt. Because of the wax or oil, the bolt will back right out, leaving the nut perfectly glued in place.