You need a frame. This ensures that the silicone does not spread uncontrollably after pouring in. You can also use empty plastic packaging to pour in the resin.
Now you can place the blank in the frame and prepare the silicone. spread the mixture evenly in the frame and let it harden. The mold can then simply be removed from the frame.
Enclose the prepared blank with the now flexible silicone. Make sure to use sufficient mass. Don’t forget that the mold must have an opening at the end through which the object can be removed.