I have two totes that I have cut out a section around and including the 2" valve and installed a steel patch with a small steel slide gate (maybe a 6" x 6" hole).  It works well to keep the mice away and store the left-over seed until the next year.  It is about as much a nuisance filling it thru the top hole as it is emptying it.  I just chain it good onto the loader forks and hold it over the drill to empty.