I am currently doing a very similar cast. My SIL works at a Starbucks while working on his graduate degree and His birthday in mid-Jan. I have already made a Latte and will post some picts of it when I finish the coffee-black pen. Mine are cigars.
ON the first "coffee black" cast, I painted the inside of the tube with dark brown and it just did not look good. I painted the next one black and it looks a lot better.
I remember you posting of the "blow up." On my first cast, the beans "floated" to the top and stood up out of the mold. The bottom of the cast had at least 1/8 inch of clear plastic and no beans in that section. For the next batch, I took some wax paper, laid it over the top of the beans and cast, put a board on top of that and weighted it down. Then vac'd for about 10 - 15 minutes then pressure overnight. I have three different kinds of plastic casts and the one that I am using for clear takes about 12 hours to set good. The Latte colored cast takes every bit of 5 - 7 minutes to set and 1 hour to harden.
The reason I decided to vac the clear is because I noticed quite a few air bubbles in the casts and since it takes 12 hours to cure, so I figured it certainly can't hurt for 15 minutes, the bubbles might rise to the top and it appears to have helped.
I have "fiberglass" that I tried also but it is slightly "blueish" tinted and decided I didn't like it with coffee.