You will need:
Various assorted screwdrivers
Drill and bits
Sandpaper or sanding block
A gumball machine on a stand. Do not try this with square gumball machines. They must be round, preferably glass.
Glass and glass cutters (or the phone number of a glass shop)
Silicone aquarium sealant (must be aquarium sealant, as bath sealant is not as strong and may contain chemicals that will harm your fish)
Rust-proofing spray paint
Push-in rubber grommets
Nuts and bolts and washers as needed
Some scrap aluminum
LED Betta light
Small aquarium filter (I used a Penn Plax Smallworld Filter)
3-plug extension cord with grounded prong
Step 1: Disassemble and Clean
Take the whole thing apart carefully, and clean everything really well. If paint is cracked or chipped, don't be afraid to sand it off and repaint it. I didn't need to. I did remove the crumbling 10c sticker, even though it would have looked nice. There was a cork ring between the steel on the top of the globe which was stuck to the glass. I scraped it off and sanded the edges of the glass down. If the steel parts on the bottom of the machine are stuck on, try to get them off. They've likely just stuck from age and pressure, and aren't secured.
Clean the entire inside of the globe with rubbing alcohol and let it dry out.
At this point I also took the stand outside to sand and paint the bottoms of the legs, which were in bad shape.
Step 2: Setting Up Your Base
Arrange your power cord, filter, and light assemblies in the base. This takes some trial and error. I tied all of the loose cable together, and ended up strapping some foam to the bottom of the filter pump to keep it from rattling against the metal. It was helpful that my light's power cable was two pieces, making it only necessary for the holes to be wide enough to pass the connectors, and not the entire light.
I nestled the connectors for the light in the candy chute, to make it easy to turn it on and off without having a cable hanging outside.
Step 3: Finding a Way to Secure the Globe
(I also flipped the bolts securing the machine to the stand, to make it easier to remove)
Step 4: Making the Globe Into a Bowl
Using your aquarium sealant, lay down a nice thick bead along the circumference of the glass disc. Lay it on to the bottom of the bowl and smooth out the sealant on the inside. Let it dry as instructed.
Once the disc is dried enough to hold (doesn't have to be fully cured at this point), repeat the process with the hold-down on the bottom and the rubber ring (if your tank has one) at the top.
Let this whole assembly dry to full cure (look at the package) before doing anything. I suggest doing this at the beginning, so you can make the most of the day or two it needs to dry.
Step 5: Set Up the Top
Lastly, squirt some sealant into the lock hole and push the lock into it.
Let it all dry.
After some time, I realized that the bare metal would cause metal poisoning in the tank. To combat this, I disassembled the cover and painted it with rust-proof paint.
Step 6: Wait
In case you're wondering, I use the pill container to organize screws and small parts.
Step 7: Water Test
Step 8: Setup
Step 9: All Done!
I quickly realized how terrible the gravel looked, and switched to all-black. To hide the tube and cord, I printed a gumball background.
EDIT: Later, I decided to spring for about two dozen glass gumballs, and dumped them in the tank for a good gumball look.