Step 1: Things required
A neodymium magnet
A bottle cap, the kind that is made of metal and can stick to the magnet.
A wood hook, the one with threads.See image for how it looks.
A hacksaw or a pliers that can cut the hook threads.
A knife to remove the rubber linings from within the cap.
Step 2: Assembly
Gas weld this hook to the bottle cap. I had access to a gas welding equipment with garages who repair dents on auto rickshaws in Mumbai.
Soldering the hook is another option. Use flux for a cleaner joint, and sand off the cap to get a real nice shiny metal. Solder will not adhere with metal that's painted or corroded.
For folks who do not, use an epoxy like Araldite or a glue gun, but I am pretty sure it won't hold much weight. Welds are best.
Place the magnet inside the cap and attach to the metal object on which you need a hook.
Spray paint it to your desired colour.
I made this hook to attach my model airplane on the ceiling. My ceiling is made of plaster of Paris, and has metal screws on which this magnet holds well.
Step 3: Measure holding strength
If it doesn't fall, add more water.
continue above steps till you see the bottle cap sliding or moving under the weight of the water.
measure the volume of water with a measuring cup. 100ml of water weighs 100gms approximately.
My magnet could hold 1/2 kg effortlessly.