By far the best cutting surface is a workbench.
How to cut corrugated metal roofing with circular saw.
Not only is it already meant to be stable and perfect for use with power tools but it goes perfectly with a cutting circular saw if you prefer to use that kind of tool over power shears.
There are several ways to cut corrugated metal.
Use a circular saw to make long straight cuts in thicker sheets of metal.
Use a pair of snips if you want to avoid power tools altogether and are working with thinner more flexible metal or need to make curved cuts.