As-fired fifty megatons- or it's as-designed one hundred?
It'd be entertaining anyway- as it enlarged the oil-spewing hole to a couple of hundred meters in diameter. :-)
I'm going to go with gathering up all of the hopelessly snarled strings of Christmas lights in the Western Hemisphere, rolling them into an enormous ball, and stuffing them in the hole.
Or something like it.
On a more serious note- I think it's already been suggested- but some sort of hemisphere with big flanges around the base, but with a valved vent on top. Like a brimmed hat. Open the vent so that oil can flow through it as you're emplacing the thing. Bury the extended base flanges in concrete blocks and then seafloor mud, then shut the vent. Bury under seafloor mud.