I've looked up a couple of tutorials for aging the bottles, and they seem really easy. The hard part would be making the larvae, but I have some ideas based another tutorial I found on making maggots from liquid latex. This will be my first foray into propmaking beyond paper handouts, and I'm quite looking forward to it.
It seems that no-one has actually made this as a prop yet, as they're such a small part of the campaign, and only briefly mentioned as a bit of a sidebar.
The description of the 'Thing in the Fog', the beast that the Investigators may come up against in the London chapter of masks is:
The Thing is insubstantial, it can be seen momentarily in bright light - a sparkling gray cloud with thin, whip-like tentacles reaching out up to 35 feet.The stats declare it to be an Other-Dimensional life form, and explain the creature as having six tentacles of mist.
The only reference to the Larvae are as ingredients in the spell required to call forward the Thing in the mist 'Quicken Fog-Spawn'. The spell is mentioned as being written in Hebrew on the Frisian Scroll. In Gavigan's Workroom, if the Investigators ever get there (or if they do, if they ever get out alive), then they can find:
On a shelf are two one-inch-long metal vials, Each bears an elder sign. Within each is a single, tiny, brittle larva - Potential Things in the Fog.Google translate gives me Quicken Fog Spawn as לגדול שרצים ערפל in Hebrew, to match the scroll mentioned (that's actually grow fog spawn, but it wouldn't translate quicken as such, and really, how many players are likely to know Hebrew?)
Of course, I was a little miffed when I read this, as I had already bough the little glass vials, rather than metal ones. But I think glass makes for a better prop, as it allows the players to see the contents, without touching them and seeing they're made of latex or plasticine.
I sketched out what I thought the larvae might look like, based heavily on a drawing here (no, I have no idea what that original artwork is for).
As a first sketch, I'm rather pleased with it, and would be fine giving that out as a handout to players. The issue may come when I make a 3D version, and try to stick it in a bottle. The mere dimensions of the bottles I picked may mean this version is too complex to be made as small as it needs to be, so I'll have to give it a second draft. Also the tentacles seem rather thin and may break easily depending on how they're made.
As a final foggy note, when looking for inspiration for this prop, I found this Fog Creature on Yog-Blogsoth, which is a great place to go looking for inspiration, as the art there is tremendous. While this Fog Creature is not the same as the Thing in the Fog, It does have the distinction of being taken from Lovecraft's Commonplace Book.