Depending on the quality of your components and the dollar amount you want to spend you might want to contact VooDoo Cables for a cryogenically treated adapter. I have found these to be very good without altering the sound. Break in is most likely required.
https://www.audiogon.com/listings/lis9334e-voodoo-cable-15-amp-to-20-amp-iec-adapter-cryo-treated-ac...
VooDoo also makes a 20 amp to 15 amp IEC adapter.
https://www.audiogon.com/listings/lis9334e-voodoo-cable-15-amp-to-20-amp-iec-adapter-cryo-treated-ac...
VooDoo also makes a 20 amp to 15 amp IEC adapter.