Put this formula in A1 and copy it across to O1, then copy A1:O1 down as far as you will want the rows to ever be filled to...

=IF(OR(ROW()>$Q$1,COLUMN()>$P$1),"",IF(OR(ROW()=1, ROW()=$Q$1),REPT($R$1,2),IF(OR(COLUMN()=1,COLUMN() =$P$1),REPT($R$1,2),"")))