I need replacement knobs for my Hawaiian breeze 20 inch box fans. The knob is on the top of the fan, not the front. I have 2 of these fans and the knobs have mysteriously disappeared.

Step 1: Measure the current (missing) knob / shaft Because there is no universal knob that fits all fans, you'll need to match a few dimensions and shaft style:

• Shaft diameter: The knob fits over the motor's speed-control shaft. Common sizes are around 6?mm, 6.5?mm, ¼" etc.
• Shaft profile / keying: The shaft may be "D?shaped" (flat on one side), double?D, or round with a flat. Sometimes there is a slot for a set?screw.
• Shaft length protruding above the panel: How far does the shaft stick out above the top surface of the fan?
• Knob inside depth / bore depth: The knob's internal bore depth must match so it can fully seat.
• Knob outer diameter / style: For grip, aesthetics, etc.

"This knob fits the Hawaiian Breeze Box Fan. The Knobs details are: 50 mm overall diameter, total of 14 mm tall, 10 mm outer diameter for the shaft, fits a 6.12 mm switch shaft. The center switch shaft hole is 16 mm deep with the D?shape the last 6.5 mm." STL Finder That's a valuable lead: someone claims to have measured a knob specifically for a Hawaiian Breeze box fan. According to that, the shaft is ~6.12?mm with a D?shape in the last 6.5?mm. STL Finder
If you can measure your shaft (perhaps using calipers, or at least a vernier, or compare with drill bits), that will help you match a replacement.
Step 2: Look for replacement knobs / rotary control knobs Here are some knobs / parts that might work as replacements. (These are generic rotary control knobs. You'll need to check the fit.)
I couldn't find a perfect "Hawaiian Breeze 20 inch box fan knob" listing right away, but here are some knobs that might be compatible (if your measurements match):
(Note: these are examples. You'll need to verify fit before purchase.)

• A generic rotary knob with matching bore and diameter
• A knob for appliance / control panel with set?screw
• 3D printed or custom printed knobs (based on the design specs above)

If you tell me your city (Nairobi? Kenya?), I can try find local suppliers or knobs available locally to minimize shipping cost.
Step 3: Alternatives / custom fabrication If you can't find an exact match, here are substitutes / fallback ideas:

1. 3D printing a knob
   The forum I mentioned includes a 3D model for a replacement knob for "Hawaiian Breeze Box Fan" with exact dimensions. You (or a maker/3D printing service) could print one. STL Finder
2. Use a universal knob
   Some universal knobs (control knobs) with matching bore and set?screw can work. You might have to slightly enlarge or shim the bore.
3. Salvage from another fan or appliance
   If you have a broken / old fan or appliance whose knob's shaft bore matches, you can repurpose that.
4. Epoxy / adhesive + knob
   As a last resort, you can glue (carefully) a knob over the shaft or fashion one out of a metal/wood knob, drilling a matching bore.