With some bulk carpet from 5.0 resto, some hand tools and some 1/4 plywood, you should be able to make something very nice and have it complete in a weekend for less than $100.
Dugan Racing has the delete kit I believe. I saw it in their "Mustang Bible II" catalog. I believe it's a fiberglass piece that you have to carpet yourself, but it looks nice.
As asked in another similar thread: Why delete the rear seat if you're just going to add weight with an amp and sub(s)? Kinda defeats the purpose.
I would say a delete should be as easy to fab up on a convertible as it is on a notch, but I'm not sure how much room you'll need to clear all of the convertible's mechanicals in back.
It offsets the wieght well offsets some of the wieght of adding the sud and amps. Plus you could get a cool hard toneau cover and make the car look like a 2 seater sportster/roadster. I was going to take the rear seat out of my car and add a stereo then the kids came and the seats went back in.