What material should I use for a custom, do-it-yourself desk for the home office

I have one in my house, never really had a problem with it. They are a bit loud though, so put it someplace out of the way - putting it in the kitchen or living room or anyplace people like to congregate and talk would not be a good idea.

The only other thing to look out for is what happens in the winter. The big problem with ours is that while the louvers close automatically when it isn't in use, in the winter heat has no problem escaping through those louvers into the attic.

It might seem like a minor thing, but it's easily noticeable - in the winter my whole roof is covered with snow except for a bare rectangle directly above the fan. They make styrofoam covers that you can place over the attic fan, but that requires you to get up in the attic every winter and put it on, then take it off every spring, which can be pain if you don't have easy attic access.

Another solution (the one we use) is to use one of those window insulating kits (with the tape and plastic, where you heat it with a hair dryer) and put that over the louvers. It works reasonably well, but not ideal.

If you could get an attic fan that had some kind of real solution for this built in - say an automatically retractable cover, or one with a pull chain, that would be ideal. Not sure if they even make such a thing, but it's something to look for.

ETA: Another thing to watch out for - make sure your attic has plenty of ventilation. Those fans move a lot of air, and you'll need plenty of ventilation in the attic to make sure all that air can get out.

300-500 lbs is not really THAT much, spread over 4 square feet. That's the weight of about 2 people. Your question is a little vague since you don't say whether this is for indoors or outdoors, whether this is a raised wood platform like a deck, or a concrete pad, or what.

If you want to use wood, you could definitely just use 2x's like for a deck. If it's not that high, you could put it on 4x4 posts and that would be plenty strong enough. You could attach your leveling feet under the posts.

To reduce vibration you could use rubber, although whatever you do you should make sure that the fan is properly secured. You can get round rubber cups that go under washing machines to reduce vibrations: that might be one way to go.