I probably made a hundred dice before I realized that there was a better way. The "Wish" molds (the little individual ones) are pretty good. They make some very sharp edged dice, but they also tend to have one raised face. This inevitably leads to sanding... hours of sanding. I'm not even exaggerating. A hundred dice, each of which has several sides to sand, takes several hour-long sessions. This is because you need to work through several grades of sandpaper grit: 320, 600, 1000, 1500, 2000 and 3000.
It was because of all the sanding that I finally decided to try something different (I'm a slow learner, it seems). I bought a bit of silicone and made a "Squish" mold, which I think is the same as a cap mold. The mold worked out extremely well. This time I used rounded dice and it's clear from the result that each die will require much, much less sanding and polishing.
I'd highly recommend doing it this way - save yourself literally hours of not very fun work, and just start with one of these. Here's how you make them:
Put the dice on some sticky-side-up tape in a mold.
Hot glue the edges so nothing can leak, then cover it with silicone.
Flip it upside down. Now cover the silicone with Vaseline so the next pour doesn't bond to it.
Wrap tape around to extend the mold and pour the silicone again to form the 'cap'.
Pour directly into the mold and voila!
Once the flashing is peeled off, you get almost roll-ready dice. This worked way better than I expected and there'll be no going back to the old way. Btw, for any of you who work with electronics, it turns out you can make potentiometer knobs with this method, too! Double win!