this should do the trick.

    public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {
         SPWeb Web = properties.Feature.Parent as SPWeb;
    
         Web.MasterUrl = Web.Site.RootWeb.ServerRelativeUrl + "/_catalogs/masterpage/CustomMaster.master";
    
         Web.CustomMasterUrl = Web.Site.RootWeb.ServerRelativeUrl + "/_catalogs/masterpage/CustomMaster.master";
    
         Web.Update();
        
         Web.Dispose();
    }