Challenge 19

Make a palisade with 5 different hexominos surrounding one contiguous area, which is to be as large as possible.
The hexominos are to be chosen from this subset of 11:

Width and height of the palisade are to be doubled and the doubled palisade is to be tiled with 2 full sets of pentominos.

The hexominos (and hence pentominos) must form a single contiguous area. The loop must be fully joined, ie not just by a vertex.
The enclosed area must be contiguous, ie it must form a single polyomino.
Example with area 19:

Do you have our excel file to draw the fence? You can download it here.

'FlatPoly2', a programm by Aad van de Wetering, can be of great assistance when trying to find a solution. It can be downloaded at his site .