NWD-balustrade

Make a balustrade with the three pentominoes, in such a way that the enclosed area is as big as possible.

Double the size of the balustrade by using a set of pentominoes.

The external perimeter must be rectangular.

The pentominoes must touch along the full edge of the square and not just at the corners.

All pentominoes must be in one plane

In the example shown (above) the enclosed part has area 17.