Overview:

My family works on a lot of electronics projects and regularly need to solder on pin headers to sensors, boards and other modules.  To ensure the headers get soldered on square, we insert them into a breadboard and use this spacer to support the opposite side. 

It is so small and simple, but it works great, and we use it all of the time.

Printing:

I recommend printing it in PETG at a minimum since you are working with high heat.  I printed ours in ASA.  No supports required if printed in the orientation shown.