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untested, but you could add all the contour points to a single array, and call minAreaRect on it:

arr = []
for x,y,w,h in contourRects:
      arr.append((x,y))
      arr.append((x+w,y+h))

box = cv.minAreaRect(np.asarray(arr))
pts = cv.boxPoints(box) # 4 outer corners