First, I got a center stop light from a wrecked Volvo V70 (MY 1998-2000), part number 9151490. It was about $10 at the local PnP.
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I was also able to get about 4+ feet of the wiring harness, which I cleaned up and reused.
I picked this light as I'm pretty familiar with Volvo stuff, and had noticed on the road that these were pretty bright, even if I wasn't directly behind them. In taking the light apart (curiosity can be dangerous

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Next step was removing a couple of ribs/mountings from the bottom of the light.
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I used a rotary tool (dremel?), then sanded it smooth.