Here’re a few possible approaches for you to consider:

1. Design Threads into the Part: Directly design threads into your part using CAD software. This method requires precise control over thread dimensions and printer settings to ensure the threads print correctly. It won’t work with all 3D printing processes, though.

2. Tap the Threads After Printing: Print the part with pre-designed holes and then use a tap to manually create the threads. This method is reliable for creating strong, precise threads and is suitable for larger threads.

3. Inserts: Print your part with slightly undersized holes, and then insert helical coil inserts or metal threaded inserts into them. This method enhances the strength and durability of the threads and is ideal for parts that will undergo frequent assembly and disassembly. The latter is probably the most reliable option.

If it has a low melt temp you can add brass threaded inserts using a soldering iron to heat & press them into the hole

Good point, thank you!