The Chernobyl New Safe Confinement took abot 8 years to build at near 2B USD cost[1]. If fire is put out, a simple geoengineering tarp with piping underneath and dirt on top will do the job. Those are used a lot in covering up old landfills to capture the gas and avoid water leaking in.

[1] https://en.wikipedia.org/wiki/Chernobyl_New_Safe_Confinement