For such a small part I suggest using a mild Acid Such as distilled malt vinegar with a small drop of detergent added (washing up liquid) to break the surface tension so it covers the surface evenly. Then leave it in a damp place and wait for a fine rust coating to develop, when dark enough apply some oil after neutralising the vinegar on the surface with some Sodium Bicarbonate or some other alkali.

The only lessons in my life I truly did learn from where the ones I paid for!