One sure thing is that the pressure of the well multiplied by the area of the BOP orifice at the top is huge. Lets say somewhere in the order of hundreds of tons. A device that equals this weight could sit atop the BOP and stop the flow as long as it seals effectivley. making the seal is difficult as rubber will just blow out and away.A soft metal such as copper aluminium or even lead could be used the trick is to locate the metal accuratley in the BOP orifice. To achieve this a spike is utilised. My design is simply a very heavy spike with a deformable collar at the end of the taper that will jam into the BOP orifice much like a morse taper on a lathe. Its weight is as afore mentioned and equal to the pressure x the area of the outlet. The only issue that I can see preventing this is the drill pipe that is stuck through the core of the BOP. If this could be removed or just a few lengths of it then this spike could work. If the drill pipe is open at its end, the spike could even thread down the drill pipe core. The spike itself could be solid or indeed a start section followed by enough ships anchor chain to give it weight followed by the plug with collar at the end. After insertion it should be chained down as shown in the diag. The spike could be made from small diameter drill pipe such that it could go inside the in situ drill pipe that is now stuck. It should be filled with lead to give it the weight required, it could be easily assembled as all drill strings are with joints.