Are you including the brackets? I'd previously taken this as just the sentence included within the brackets but not the brackets themselves. I've obviously seen brackets in plenty of fiction, but I think it'd seem a little out of place as there'd be no other occurrence of that type in my story. "This is it now, Isaac," he said through his teeth. "The last thing we want is to give ourselves away." He gave a momentary shudder. "When we're trapped in a box." The above is really what I thought you meant with the first example, that is with brackets excluded. Your modification of ellipses would work well to keep fluidity, but I'm not sure if it needs brackets to be grammatically correct or not.
