Last year, my kids gave me a set of flexible silicone molds to cast my own bolt heads and rivets in resin. It's essentially an 8x10 piece of rubber that has lines of cavities of pre-determined sizes ranging from 0.25mm up to 2.5mm in diameter. Whenever I cast pieces, I always seem to have a little extra resin mixed up in the cup and I hate to waste it, so I keep the mold close at hand. Essentially, you just sort of drizzle the spare resin over it, trowel things off to level it, and flex the mold to release the pieces once the resin has set.
I don't have the literature that came with it, but I think that the manufacturer was Greenstuff World. They do a lot of similar molds to let you do bricks and masonry, as well as structural bits and greeblies for sci-fi builders.