I find that sometimes the CB silicone washer does the better job and sometimes the orange silicone o-ring. Can never really put my finger on why, maybe one works better on a flatter surface or something - so if you get them from there, get one of each.
Yeah the pot is thinner than the wall of the kettle so the screws bottom out when you tighten them up. Either use shorter screws (M3 I think) or pack it out with something, could maybe cut the element hole section out of the jug? I just cut a packing washer out of 3mm ply