My friend had a convertible 1st Generation RX-7 for a while. He bought it already customized from a guy with lots of experience doing that stuff. Wish I had a picture of it, it was really a sweet looking sports car.
Here's a few things I remember about it:
Structural reinforcement in the frame to handle the lack of roof.
A sporty looking rollbar system welded in.
Since there was no longer a hatchback, it had a trunk, the lid being the sunroof from a Toyota Camry.
Totally custom vinyl top with rear window, that retracted manually into the rear very nicely.
A cut off switch for the fuel system was hidden under the center console so that the car would not start even if it was hot wired (under the assumption I guess that convertibles are even quicker for a car thief to break into).
There was also a remote release for the trunk hidden where the cut off switch was. So you could secure valuables in the trunk and therefore leave the car doors unlocked, leaving no temptation for a thief to rip apart the vinyl top. Let them steal the $50 tape deck, better that then pay for a new top.
Aside from the custom stuff, it had a really nice silver paint job that faded from light silver to a dark grey. From a distance the fading gave it a stainless steel look.
Also had some nice custom rims and of course some engine work (as if those cars weren't fast enough already). The thing was a suicide machine.
At one point we kinda stupidly put the structural integrity to the test by having three of us big guys in it (this was when we were young and dumb). Between the three of us we probably had 750 pounds in what was stock, and still was, a 2 seater (by not having a top, there was actually room behind the front seats for a third person if they kinda crouched down and held onto the roll bar, not for public roads though). I was the lucky guy stuck in back sitting on my heels, and when Jason slammed it into gear I was almost flung out the back onto the road. I held onto the roll bar for dear life as he went throw the gears, burning rubber in at leat the first three gears.