Close helmet farming

I thought I knew how to farm white close helmet to chance them, apparently I don't...

They are level 26 so I thought I could go in marketplace which is level 26 but after 40 minutes I have dropped none.

Is there any "efficient" way of farming it? I want to at least chance all the close helmets I can before burning 200 chaos on the baron.

I was on docks, after 5 runs only one dropped so far...

They drop pretty frequently in the normal lab. Seems to me like the most efficient way to farm them. I normally get 3-5 per run.