Free form Text field reaonly in configurator

We have Free Form Product property. User should be able to enter any value in configurator and save it.
For some conditions we want to make it ready only field(field greyed out). How to make greyed out? Is there any workaround? Right now since we are unable to make it greyed out, we had to write a constraint rule for that field. But downside is user is still able to enter value but as user exits the field, constraint rule is invoked and forces user to undo it. We are able to stop the user on exits, but we dont want to allow user to enter any value also.  Please advice if there any solution to make field read