[dennisw]

Curing concrete takes too long. You could pour a huge steel or iron block or cap and use it within days.

Say a 30ft x 30ft x 8ft thick. Drop it on the mutha.

[NVDave]

Curing concrete need not take weeks. Concrete curing can be accelerated with additives. Depending on the structure, you could have something usable in five to seven days.

Then there is hydraulic concrete. It hardens under water.

[Tex-Con-Man]

Went looking and found some interesting info.

BP has completed what they are calling a “containment dome”. A 4-story, 70+ ton structure of steel and concrete.

Here's some articles:

http://www.nytimes.com/2010/05/03/us/03spill.html

http://www.nytimes.com/2010/05/04/us/04spill.html?src=un&feedurl=http%3A%2F%2Fjson8.nytimes.com%2Fpages%2Fnational%2Findex.jsonp

This one has a drawing:

http://alfin2100.blogspot.com/2010/05/containment-dome-in-6-8-days-blowout.html

http://www.terradaily.com/afp/100502144910.mr9ysucj.html

http://thehill.com/blogs/blog-briefing-room/news/95475-bp-executive-company-will-deploy-containment-system-for-spill-within-week