HI!  
    Your  best  bet  in  fixing  your  problem  is  to  do  it  rigth. 
   You  should  cut  the  pice  of  metal  with  a  grinder  and  weld  
   a  new  one  in  place. As  you  may  know,  disimilar  metal  cause  
   corrosion,so  even  if  you  place  a  pice  of  metal  over  and  
   sealed  with  B 1\2 (sealer  use  for  aviation  aplications)  you    
   may  still  develop  some  corrosion.  If  you  dont  heve  too  much  
   money  or  a  friend  with  a  welder  you  my  clean  the  area  
   very  well  and  lay  a  pice  of  ply wood  and  keeped  clean.
   Were  are  you  located  I  may   help  you  to  do  some  work  if  
   you  desire.
                                                     A&P
                                                 BUG  MANIAC