How I harden off plants is by shaking them at the base of their stalk every morning. If you shake em a bit (5 seconds each) then they will grow thicker stalks. I also have a fan blowing on them. I like them to shake a bit but not have leaves being blown around. I also place mine outside for a couple hours here and there. I like them to get a little bit of good natural sunlight and they also get "hardened".

So a couple shakes everyday keeps their stalks growing thicker.