There's almost no AFK or even low focus methods out there, except for one.
In the Nightmare Zone, if you choose monsters that drop big bones upon death, they have a chance of dropping long or curved bones, which you can exchange in the Dorgesh Kaan City for loads of Construction XP.
In terms of bosses that drop big bones, there's the Skeleton Hellhound, Dad, Arg and the Black Knight Titan, and when you do get a long or curved bone drop, it actually stays on the ground for 10 minutes, which is longer than the standard 3 minutes.
You might be thinking that these bones are really rare, so is there much point in doing this?
Well if you're going for max combat stats in the Nightmare Zone, you can expect to be there for at least 250 hours, and in those hours, you'll get at least 200 curved and long bones, which translates to over a million Construction XP, completely for free.