I've just tried this and it worked pretty good, eventually.
I had a small part I wanted to replicate and thought of this thread. And I had some old tubes of silicone laying around. So white acrylic paint and glycerin was added to soapy water and some silicone was squirted into it.
It made a nice blob which was smeared on the part. The piece was made of a waxy feeling plastic so I wasn't too worried about it sticking.
However, after a few hours it was still sticky to touch. I squirted a bead onto paper and it didn't dry either. Obviously the silicone had gone bad.
FFFFUUUUUUUUUUU....!!
So after I cleaned off the part it was off to the store.
Then I made the same mistake as the OP. After looking at several clears I bought a tube of white. It wasn't noticed until it was squirted out. Now I had white silicone in white water.
This time I also forgot the soap so my hands were covered with silicone.
FFFUUUUU..!
After using many paper towels it was cleaned off.
But when I poured out the water the bits of silicone remaining were already set! I touched the piece and it felt dry as well. I left it for several hours anyway, and then it peeled off easily. However the thread pattern wasn't clean. So I made another mold.
This time I put dish soap on my hands and let it dry. This is a trick used in automotive work. Very little silicone stuck to my fingers, even though I worked it in and shaped it better.
I made the copy with fiberglass and resin. Some baby powder was added to resin to thicken it then it was smeared into the thread pattern. Then cloth was added.
When it cured a bit of sanding cleaned it up. The threads screwed right in and sealed.
So what is it anyway? It's the nose piece for this.
I combined a used gasmask with some welding goggles. Something like this may cost 500 bucks new. Mine was close to free.
I wanted to try pumping low pressure air into the nose to defog the lens and help cool it down in summer, but didn't want to damage the original nosepiece.
[ I also read that tube silicone could be thinned with paint thinner or Xylene. I tried xylene, and it became pourable, and still set normally. It captured some fine detail on a coin, but was still not as thin as a commercial product. It would require a lot of thinner. Perhaps twice the silicone by volume.]
For projects like this the silicone caulk is ideal. I used about 1/20th of a 5 dollar tube. The rest will last for years, or can be used as caulk.