Grand Idea, no experience

I'm considering trying my hand at casting light lenses for a vehicle I'm restoring.  It's a 1955 Packard Constellation, and the first lens I want to try is the backup light lens.  It's clear, but has striations along part of the inside, and what appears to be a magnifying circle in the center of the lens.  My big question is that the lens itself is relatively simple, but it is somewhat "V" shaped which requires a large mold.  Rough measurements for the mold requirement is appx 7" from the point of the V to the rear, open end, and the open end is appx 7" across.  It measures appx 3" high.  According to the calculator, I would need almost 12 pounds of silicone to make the mold!  My question is "Is it recommended to either use some sort of "filler" in the hollow area, or make an odd shaped mold"?
I've perused this forum and watched some of the videos and was wondering what would be the recommended products to use for this project?
If this works well, I will next attempt the tail light lenses.
Thanks in advance,
John