I use Oramask 813 Stencil Film to as a sticky base. This prevents the object you are molding from floating up. It sticks really well and does not leave residue when you remove it. Remove any dirt or oil from the object before you place it, to ensure it sticks. You can buy it on amazon here:
https://amzn.to/3q8p1fh. If this technique fails, get out the hot glue! For the bottom part, you can use many materials, such as clear shipping tape, laminate sheets, double sided tape, etc. I strongly suggest using HOT GLUE around the base for bigger molds.
