I need a extension cord with

this won't break the bank

2 from 1

Given some recent recommendations I’d reiterate the OP’s requirements that include no power conditioning and 20 amp compatibility just to avoid future wasted effort/time.

I second @audphile1 and the suggestion to simply a change your duplex box to a quad box.  Use whatever fancy (or not) outlets you want.  Depending on your current draw needs, service box capacity, and access, you could even run two dedicated lines to the two outlets, or not.  Either way, the most practical and least invasive(sonically) solution, IMO. Good luck.

500 for a quad box with a short cord and a plug are you nuts ? And noise and or sound altering from the cover… stupidity ! Please get an electrician put a quad box where the duplex is nobody in their right mind will notice the difference, you’re talking inches of drywall altered. Unless a person is using high draw, high output amplifier’s, there won’t be a problem ! The first question should be what type of equipment and demand is predicted.

@curiousjim   Get yourself over to Home Depot and buy a quad outlet box, a couple good outlets, a few feet of 10 awg wire and a 20 amp plug.  If you're worried about a metal top plate sucking the sound out of the music, buy a plastic one.  The whole thing should cost you less than $20.  Try it out before you spend $500 on something else.  You know how your system sounds now and should be able to tell if it still sounds the same.  Gamble the $20 and report back with your findings.  We are always preaching on this site to "use your ears".  I suggest you follow that advice here.  Cheers