Will a cheap RFID reader work behind a 2mm aluminium sheet?
You'll find that it won't work, the metal will block the signal from your standard cheap RFID reader completely. Drilling holes will make absolutely no difference.

You would need to cut out an aperture the size of the RFID antenna plus a decent margin (20 mm or more all round?) and mount the reader within that aperture or on the RFID tag-side, not the 'other side' to where your tags are.

One option to consider is putting your reader in a tough plastic box and mounting that on the tag-side of the metal wall, with a hole drilled though to take the cable. Make sure the plastic box keeps the reader inside away from the metal by at least 20 mm, preferable 40 mm or more. You'll find out best by buying one and mocking this up by your wall.

A couple of years ago, I spent a good amount of time investigating RFID technology for use in a machine in a large metal enclosure. One part was me experimenting with a commercial (£30) USB reader and how it would read through different materials when near or surrounded by metal. Was interesting stuff.