I'm sorry for the sarcasm, but what do we want to do here? We all need to get you to cut a hole of some fashion for a fan. And it has to be done with some sort of tools beside the only mentioned tool: the drill bit.
We also want need to talk about how to go about doing so simply, and cheaply. I just don't think snapping back with "octagon" was the best method of progress in your quest. Ain't mine. I'll cut holes all day. I'll even do a couple Octagons, left handed, just for kicks
However, it isn't so simple if you don't have either of these available. Making a round hole just takes some patience. So a more simple idea would be to make a square hole instead of a round one. Again, just making sure the square is smaller than all points of the fan's shroud/frame.
I really don't know how to tell you how to cut a nice round hole without A) the perfect tool for one-shot/one-kill B) an eye for cutting a good hole, and then patience and the artistic ability to finish out the hole with a file.
So I mention a square hole.
Either way, you gotta have a tool to break steal You're not going to cut any hole 20mm and above with drill bits. Modding isn't just having the idea. It's doing it, and we're all working with materials we can't form by hands alone.
I'm a tool guy. I spent my money mowing lawns on Snap-On chests and hand-tools. And that was 15 man. It's relative. I wasn't in this computer modding back then, but had tools for the long haul and could cut a hole. To be specific, I bought those punches when I was 15 and 2 months short of graduating HS. Got them through the machine shop vendor that came by every month. Age to have tools? I gave a 3yo a cresent wrench this year.