Many of the large hard swimbaits have a soft plastic tail which can be easy changed on the water by pulling out the finishing nails that hold the soft tail in the bait.

Most of the bigbaits I fish have adequate tails, but sometims they do not come in the color that I want, or sometimes I like to experiment with different size/shape tails. Making a silicon mold, and then not liking the tail is an expensive, time consuming process.
Here is what you can do to make a cool tail, with nothing more than a cup full of plastic, a sheet of white 8 1/2 by 11 paper, and a Sharpee.
When cool, and I forgot to take a picture of this, cut the plastic around the paper insert. Leave about 1/8 of an inch, all the way around.
Heat up another cup of plastic, and pour out again on the counter. Now your drop the plastic coated tail in the hot plastic,

and cover with another layer of plastic. This means you have two layers on the top, and two on the bottom.
Cut out the tail again, leaveing 1/8" all the way around.

Cut out the shape of the tail that you want from the white copy paper. I did not even draw this one out--I just cut it like a square trout like tail.

Ultilizing my 3rd grade level art skill, I marked/colored the tail with a pink Sharpee. I tried to make the tail have fin-rays, but feel a bit short.

Heat up a cup of plastic, color of your choice, and dump on a flat metal or glass countertop. You can also use tinfoil--the plastic will not stick to it when cooled

Drop the paper tail flat on the still hot plastic.

Pour a generous layer over the top of the paper. This will essentially seal in the paper tail between two thin layers of plastic.
When cool, and I forgot to take a picture of this, cut the plastic around the paper insert. Leave about 1/8 of an inch, all the way around.