Draw lines down the edges then call imfill, then erase the lines
row1 = find(binaryImage(:, 1), 1, 'first');
row2 = find(binaryImage(:, 1), 1, 'end');
row3 = find(binaryImage(:, end), 1, 'first');
row4 = find(binaryImage(:, end), 1, 'end');
% Seal edges
binaryImage(:, 1) = true;
binaryImage(:, end) = true;
% Fill in the line.
binaryImage = imfill(binaryImage, 'holes');
% Erase temporary lines on edges that we made.
binaryImage(:, 1) = false;
binaryImage(:, end) = false;
% Fill in left and right edges.
binaryImage(row1:row2, 1) = true;
binaryImage(row3:row4, end) = true;
