A tongue and groove system could be used to seal the edges and where the flat sides come together. The size of the panels would be geared to the capacity of the delivery machinery available. Would 10 x 10 x 2 ft be too heavy to put in place?
The result might be a partial pyramid, possibly 100 m2 with several interlocking and overlapping layers, as many as are needed so their weight exceeds the force of the gusher, in the same way that we are told to apply pressure to an arterial bleed, for example. The seabed might need levelling though, unless custom cast pieces can be created to fit to the existing topography.
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An alternative approach might be to lower one absolutely massive piece of concrete onto the leak but such a heavy piece might not be manageable, in which case "the one piece" could be put together lego-like by lowering it down it in interlocking sections each on top of the previous - a bit similar to the tiling plan but with much bigger pieces over a smaller area.
Is it not the case that if you pressed a gusher like that forcefully enough that it could not continue? Why would this not work?