You might consider using IB's EFPs for unused funds, if you haven't already.Quote from SideShowBob:
It is generous of IB to not pay us negative interest (e.g. charge us) for our unused funds.
Also nice of them to add mutual funds but avoid any and all money market funds, so you have no chance to get any return on those funds.
Quote from J.P.:
You might consider using IB's EFPs for unused funds, if you haven't already.