[Beagle8U]

I would use a solid hard rubber plug, undersized enough to fit freely deep into the well casing and allow the high pressure oil to escape around it. ( otherwise I think the pressure would be to great to get the plug deep enough in the casing ).

On both ends of the, several feet thick, plug would be a heavy plate steel washer.

Running through the center would be a large threaded rod.
( acme or buttress thread to prevent stripping under pressure) Like the threads on a bench vice ).

A large nut would be fixed solidly to the bottom plate, but the upper plate would would be topped with a nut that could be tightened from above to squeese the plates togther and expand the rubber plug to seal the casing enough to cement it shut.

I think that would work.

[old curmudgeon]

Too practical.

The fact that this is a common technique makes it too sensible.

As a matter of fact, I own a large animal veterinary syringe that has an adjustable plunger that works exactly as you describe.

A very good suggestion. And one that could be manufactured out of ordinary materials on a crash basis.