The backup lights built into my tailights did not have any reflectors. The back of the light housing was the right parabolic shape, but it was black.
It produced 550lumens per bulb at two feet, basically a large candle.
I bought cheap led replacement bulbs and then lined the cavity with aluminum tape like you seal ducting with.
All I did was cut two strips and put them on the back in a cross pattern. No cutting circles or filling gaps.
I'm now at 5500lumen at two feet, per bulb.
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