Sure, Clark liked to help in secret prior to coming to Metropolis.
We all know that in L&C canon he invented Superman from remarks Lois made about a spare suit of clothes.
However in this day & age of computer software a maskedless hero would be IDed fairly quickly.
What if instead Lois made a remark about the oldtime radio show The Shadow and the Pulp magazine stories?
Clark adopts the existing costume and background of The Shadow. Even if someone does see him the witness is discredited by his own words.
A well done human shadow face mask would work because Clark would not need eye holes, and with the mouth hidden by the scarf no one would notice it does not move. His voice would be distorted and his age hidden. Only those who got really close would see it was a mask.
The Shadow used a number of identities some belonging to living people he borrowed with permission, some created.
Would a fire chief have any real problem with The Shadow if he still saved lives? If he still did Superman stuff and sometimes the hat was missing folks would still think he was appearing without a mask. Only those very close would know it was a mask. Perhaps Clark could have a "supertech" face mask that even up close looked real? And with the the nose and ears normally seen on The Shadow no one would suspect Clark.
If anyone did think it was Clark it would appear that The Shadow borrowed his identity briefly.
