This was the simplest method I could think of to make a holder/insert press for my cheap soldering iron.

I "think" it will fit most small  soldering irons, the taper of mine is 18.5 to 20 mm diameter over a 23mm length of handle measured to the outside of the silicone grip.   The grip gives a little flexibility.

Use 15mm  M3 screws and nuts.

I have used a 12mm aluminium tube offcut to insert into the drill chuck, a timber dowel or even a drill bit will suffice, and there may be enough flexibility in the print to fit a ½" (126mm) rod.     

I've printed this model in PLA (Galaxy Black Prusament) with standard PrusaSlicer “Balanced” settings and it has had no deflection at all in use.

The only disappointing thing is that it is so efficient that my last job inserting 75 3mm inserts was so efficient that I almost didn't have time to enjoy the perfect results every time!