Look for sheet metal shops in your area.
I walked into one and they cut and gave me 2 24" x 30" sheets of scrap for free.
I had to butt them on the board - no problem.
I was looking for a piece that is used in HVAC.
These are galvanized and won't rust.
Don't paint whatever you get, the magnets will scratch it to hell.
Instead, wax it periodically with car wax.
Or go to a woodworking supply company and get what they use on table saws.
Good Luck,
KW_Counter