Hi,
You might be able to make a clam mold with RTV, but theres a few concerns to look at first.
#1 you mentioned your RTV has been sitting around awhile, you might wanna test it to make sure its still good. RTV has a shelf life, usually around 6 mo. but depends on brand & how you store it.
#2 durometer. in the clam mold tutorial I use a typical household silicone. it has a high durometer/low shore value which basically makes it "thick"
most RTV's are pretty flimsy so you might wanna sink the bait a little deeper than the 1/8" I recommended in the tutorial.
The RTV wont be thick enough to support the bait, so you will need to either suspend it from above using something like a thin wire or a needle or something. I called it a clam mold because its totally encompassed by silicone.
#3 The clam mold is made from clear silicone, so you can see where you need to cut the mold to extract the original, I havnt seen many "clear" RTV's & theyre pretty pricey, So finding where to cut might be tricky.
Other moldmakers will use a method called a "glove mold" where one end is open & creates the pour hole. heres a link to a similar tutorial for more reading:
http://www.alumilite.com/howto.cfm?ID=7
On Step 4 of this tutorial the author uses a marker to make note of where he needs to cut the mold, I recomment you mark the Inside of the container so the mark will transfer onto the RTV rubber, ive done it & makes it easier to find the mark.
I used a glove mold to make my swimbaits, I stood the bait on its flat tail to make a glove mold described in the link above.
hope it works out for ya.