Since he's not going for the all popular bars, I'm guessing his hammy senses tell him the best escape route is through the plastic roof
When I see hams start a gnawing habit, I try everything to break it ASAP, but I noticed that it's different when they just mindlessly gnaw on a part of the cage, and when they find things they move or loosen or break when they gnaw it. If I put Ziggy in a tiny cage to clean his luxury condo, he clearly chews to get the door open...not just mindless chewing. Since he manages to get the door open partially....
I'd prevent the ham from getting near that place then...removing or lowering the second level, or just making it impossible to get at the top part....