How to mount electric box in screen porch?

We had a screen porch built. The ceiling is insulated, and one ceiling panel has a 4" "fan channel" that runs the length.
We bought an exterior ceiling fan, and I'm confident I can run the wiring (tap off of existing exterior light) to the fan and actually install the fan.
The only thing I am not sure of is the electric box I have to mount. How would it mount to the ceiling? The ceiling panels are supposed to support quite a bit of weight - I weigh 230 lbs and can walk across the top no problem.
The electric box I bought is for a ceiling fan and has three holes in it for mounting to surface. It included lag screws but those are for a wood joist, correct? So what hardware would I need to mount the electric box to the ceiling, and how to go about it?
Thanks, - snipped-for-privacy@aol.com
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Correction ... ceiling has a "fan beam".
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Pictures please.
Fan boxes must be supported by the structure. Fans can not be mounted below an 7'6" ceiling, even huggers here abouts. Huggers really do not do much anyway.
Check out Wiremold or Walkerduct. They make exposed wiring systems that do not look half bad when painted.
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This is the kind of question the manufacturer can answer. Ask the guys who sold you the screen room. The screen guy will probably say to just use their regular drill screws into the metal
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