[kdv]

I wouldn't use virtualbox in any situation where security is a concern nor Qubes OS unless you know what you're doing. Your best option for minimal effort is to use VMware workstation or Fusion, avoid shared folders and transfer data using SSH over a host-only interface that you disconnect when not in use.

Bonus: If you don't need two-way transfers, you can even setup a one-way transfer system that only allows the client to write drop files to specific directory.