All you need is plasticene (modeling clay), various sizes of small plastic tubes, and some latex rubber (available at craft stores). I began by scooping out a channel in the clay in the shape of the tentacle, with one end larger and narrowing to the other. I then took the tubes and carefully poked shallow indentations along the surface of the channel to make the suckers, the larger tube at the larger end getting increasingly smaller as I reached the smaller end. I was working with three different diameters in my tubes. Metal tubes are too narrow, which is why I suggest using plastic or rubber tubes.
Now I had my mold, or negative tentacle. Next I brushed a coating of latex rubber into it, carefully blowing any bubbles so that the liquid rubber would sink down into the indentations. I then brushed in a liberal coat of rubber so that I couldn’t see the individual suckers anymore. I let it dry overnight, and voilà, a quickie tentacle! Okay, it’s only half a tentacle, but in a specimen bottle I think it will look good.
Above is pictured the finished latex tentacle as it came right out of the mold. The photo of the clay shows it after I demolded the tentacle, so the clay is somewhat distorted. The latex rubber leaves a beautiful feathered edge. It can be colored by simply adding a little latex acrylic paint to the liquid latex rubber before brushing it on. Of course the tentacle can be trimmed with scissors, and if the original mold in the clay were done more carefully I think a really spectacular tentacle would be fairly easy to make.