In a pinch could I use silicon caulking (the kind in a tube for sealing around bathtubs, etc) to make silicon moulds?
Depends on if it hardens enough
AND
if it can be unstuck from the original without ripping the mold apart.
For simply shapes you could use the latex brush on mold material used to cast
ugly little plaster figures.
Depending on the shape and size of the mold it may be necessary to use a plaster (or other hard shell) backing to support the mold.
Sharp undercuts doesn’t work well with this method.
Someone testing the silicone caulking material:
I’ve used silicon, and RTV mould making kits in the past, but they’re expensive now, and with no local hobby shops I would have to order on-line, and shipping makes them even more expensive! However, there are several DIY stores that carry all kinds of silicon caulking. Probably be cheaper than ordering a mould-making kit.
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I have attempted this myself for small parts, and the main issue i had was the stickiness of the silicone made it difficult to get it to form around the part without air bubbles.
One solution i found online is to mix it with corn flour until it turns to a kind of putty consistency. Makes it much easier to work with and less sticky though I would still recommend mould release on the part. Give plenty of time for curing as it’s much slower to cure than purpose made moulding silicone.
It’s been a while since i tried, but i think i also remember it needed acetic cure caulking(the smelly stuff) to work properly, but not sure.
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Yes.
Apply thin layers of no more than 1/8" at a time. This is air cure material so each layer needs to cure before adding another layer. For example I molded a one sided part for a hub cap using gasket making silicone material from a tube.
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Thanks for the replies. So it is possible; not perfect, but possible. 
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As the old saying goes:
Try it, you might like it …
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There are some decent mold making kits on Amazon that I have used to great success and are not overly expensive.
Cheers Rob.
Hmmm…looks interesting and inexpensive! Thanks! 
Walmart carries the Alumalite kit. It is only about $25-$30 and works great. It cures and is ready for use in about 30 minutes too. They also carry the epoxy resin that goes with it.
Yes. I talked about it on the old forum once. In a pinch you can even make the part itself from hot glue.