You should the logic of taking the max area of rectangles as shown below. You can use this snippet to get the mask image of the area:

import cv2
import numpy as np

image = cv2.imread("jigsaw.jpg")

image = cv2.resize(image, (image.shape[1] // 4, image.shape[0] // 4))
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray, (5, 5), 0)

thresh = cv2.adaptiveThreshold(blur, 255, 1, 1, 11, 2)

contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

max_area = 0
best_cnt = None
for counter in contours:
    area = cv2.contourArea(counter)
    if area > 1000:
        if area > max_area:
            max_area = area
            best_cnt = counter

mask = np.zeros((gray.shape), np.uint8)

cv2.drawContours(mask, [best_cnt], 0, 255, -1)
cv2.drawContours(mask, [best_cnt], 0, 0, 2)

cv2.imwrite('mask_jigsaw.jpg', mask)

cv2.imshow("Image Mask", mask)
cv2.waitKey(0)

Input image:

Output image: