16 g is pretty brutal for a steel patchbox. I like 18. I used to use 16g but as my skills developed I have been moving on to thinner and thinner material. A flat lid with no dome is not as rigid as a domed lid, therefore, thinner material works easier and is just as strong after being domed. When I dome out a lid I use a home made hardwood punch about 1/2 to 3/4" wide by 1/4 to 3/8" thick with a slight radius the matches the dome. The wood leaves the surface much smoother than all the dimples a hammer leave which then must be smoother out. Once the lid is domed up I go around the perimeter of the dome with a smooth section of flat bar to sharpen up the bend.
David