Here's where everything starts to come together. Assemble two of your mold shells with 1/4-20 bolts. If you rescaled your tentacle, you'll need to pick some fasteners to fit the smaller holes. Put the core in the mold and check the aligment. If the core collides with the mold walls or touches in the middle go ahead and trim it down with an xacto or rework it with your soldering iron.

Check the seams between the pieces for gaps or cracks. Chances are you'll have to put a bit of clay or putty anywhere that doesn't fit super snugly. It's one of the worst sensations in the world to spend a lot of time and care prepping and pouring a mold only to realize it's hemorrhaging silicone everywhere.