Following Henrys advice I also had good results with using Viton-rings, Dow 55 and chamfering the case. In order to avoid spinning the washers (together with the O-rings) I used a clamp and a piece of wood to apply enough pressure to „squish“ the O-rings into place. Bolt head can still be reached with a thin open-end spanner to apply enough torque until the washers will not move anymore. The opposite washer can be kept from spinning by hand. Of course the clamp will have to come off for final torquing.
