I need a extension cord with

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

Furman SS-6B-PRO 6-outlet Pro Surge Suppressor Strip ...look at the reviews. I have 3 of them. You are extended and protected.

DIY example, pricing. YMMV.

2-Gang Aluminum Weatherproof box, 3/4" hub $11.98

3/4" cord strain relief connector $4.25

Hubbell 5-15P hospital grade plug. $23.50

12/3 rubber cord sold by the ft 5ft, $10.25

15 Amp Industrial Grade Heavy Duty Self Grounding Duplex Outlet, White $10.61 This outlet also comes in gray.

15 Amp Duplex Receptacle/Outlet, Industrial Grade, Self-Grounding 5262- Specs. Very good outlet. Non plated brass contacts. Solid brass supporting back strap. NOT galvanized steel.

Legrand® Pass & Seymour Trademaster® Gray Nylon 2 Duplex Outlet Wall Plate

$1.76

Total $72.96 + tax.

Misc items $5

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Here's a video that shows how to convert a single wall outlet to two or more. If you are handy with tools and comfortable with wiring, it's really not that big or difficult of a job for a DIY project.