Ok you lost me.. A 20 amp cable is a 20 amp cable, the ENDs reflect that. A 15 amp cable is not the same.. Is that the mix up.. You're trying to install a 10 or 15 amp cable into a 20 amp female?
Just get a 20 amp cable, nothing fancy.. I would get a good cable for that amp though.. No question on that.. Don't cheap out on materials.. Cost that's a different story.. Depends on who makes what?.. 50.00 usd to 5K pretty easy..