I woud take your 5mm and use it as a core (from the middle point of your sheet out a distance in each direction) for your normal 8mm sheet then to a locking stitch down each side. Distance should be at least two fid lengths, locking stitch the full length. There may be better ways to incorporate a brummel splice (which refers to just the lock that you see in the above link, not the loop) so that it is actually locked.