I would imagine just "putting a cylinder" over the leak would not contain it. To begin with, the floor of the ocean isn't a solid surface, there's a lot of mud which means the opening to the hole isn't "clean cut" - any kind of "well" uses an inner sleeve so that you can connect a pipe to the "hole". That's usually where the blowout preventers would go, I imagine, would be in the sleeve inside the hole. So, the opening to the well is probably pretty muddy and unstable.
When that well blew out it popped off the connecting pipe and began releasing thousands of barrels of oil at very high pressures. Even if the "hole" were clean cut and they could easily mount a fitting to it, the pressure alone would make it extremely difficult (try turning the garden faucet on high and then connecting a hose to it, possible, but rather difficult).
This is why blowout preventers exist, if a blowout happens the well is sealed from inside itself (as I understand it). I think it is ridiculous that mandatory blowout preventers never made it into law. Bright side of it is this incident will (or should) inspire reform.
I think everything they are doing is about as much as you can do at that depth with such high pressure.