One additional note: for best results with large objects I recommend designing your object for use with a particular glass, the reason being that your object will float with the addition of liquid UNLESS it fits perfectly snug into the glass (of course it will eventually melt, but will look totally awesome for a short time :).
You will need:
(1) A 3D printed object
(2) Food safe material for mold. I used Smooth-On's Smooth-Sil 940 as well as 1/8" Polyethylene sheets from Tap Plastics
(3) Release Agent
(4) Plastic Container & Stir Stick
(5) Glue Gun
(6) Nitrile or plastic gloves that ARE NOT latex
(7) Scale (Smooth-Sil 940 uses a material ratio of 10A:1B)
(8) Sandpaper or Sanding block (grades 180, 220, & 400)
(9) Clean surface
For intermediary objects (explained later):
(1) Cheaper silicone than Smooth-Sil, such as Mold Star (which de-airs itself)
(2) Smooth-Cast 325
(3) Plastic Container & stir stick in addition to relevant items mentioned above
Recommended: Pressure tank and compressor
If you decide to make a tray as opposed to a mold you will need access to a vacuum forming machine. Alternatively, you can build one: check out this great Instructable.
Note 1: Thanks to HexCorp in Tarzana for letting me use your space and 3D printer, a great makerspace is emerging! Thank you to UC Santa Cruz for continued sponsorship as a research associate and owning a vacuum forming machine.
Note 2: I am in no way affiliated with Smooth On, Autodesk, Makerbot or any other company mentioned in this Instructable and receive no compensation for the publication of this Instructable by either of the aforementioned companies.