You will need:
Gumball machine: I prefer round, glass globes, but it's up to you.
1/4" thick glass, cut to size, with polished edges
Aquarium sealant. Regular caulk may contain chemicals harmful to fish! Don't use it.
Small tank filter
Colored glass marbles (optional)
Assorted threaded rods and nuts
Foam (for noise dampening
Rubber grommets (assorted sizes)
Jigsaw (or a lot of patience with the hacksaw)
Razor blade (for trimming dried sealant)