What layout for a nearly empty web app?

Context

I am designing an ad hoc enterprise web app that is very simple: given two parameters (year, month), click a button that generates and let you download a file (an Excel spreadsheet). Users are data analysts, and would only use the app couple times a month. The app is to be accessed on a computer, via a web page.

Problem

This makes that, beyond header (logo, app title) and footer (legal and other corporate one-liner blurb), the app is constituted of only 3 elements: 2 drop-down menus, and one button.

As the corporate style-guide is flat-design oriented, there is a lot of empty space on the page and it feels too roomy and quite wrong — even with already fairly large font-size

Question

How to make a nearly empty web app not look like an empty apartment?

I have looked at very limited interface apps like WeTransfer, but they have (at least) twice as much elements, and their strategy to “fill the void” by using a picture wouldn't work in a corporate environment.