Export data as .xls or .xlsx

I have a script that grabs data, processes it, and exports the results as a CSV. Cuz CSV is EASY.

But it's only a 4-column CSV, and by default, Excel displays really narrow columns that need to be resized to see all the data. Bottom line, I'd really prefer to export this data as something more robust than CSV.

Now, as far as I'm aware, there's no spreadsheet equivalent to RTF, is there?

How feasible would it be to actually get this data into a formatted Excel file?

I'm aware that .xlsx is basically a bunch of zipped XML files, so I'm currently trying some reverse engineering there, I just didn't know if I might just be spinning my wheels.

Thanks.