What is it made out of? If it is corrigated sheet steel or aluminum, I would reccomend a sawsall and a metal blade. You can drill a couple holes in a row with a drill and connect those together with a chisel to make a slot to insert the sawsall blade initially. You can also start out the slot with a angle grinder or a die grinder with a cutoff wheel to get the slot to insert the sawsall blade...
Sounds Crazy, When I was building a 2 story metal building for barracks in Saudi Arabia I cut 20 window openings with a wood blade turned around backwards in a skill saw. Its not to hard to cut pipe culvert.
If it is Galvinized and you use a O/A "SMOKE WRENCH" Don't breathe the fumes.
Wear a face shield and hearing protection and Go For IT.
Go with the Sawzall and the metal blade. I've tried using backward plywood blades and metal blades in a skill saw and picked up more shrapnel than I did in Viet Nam.
Well if you happen to have a plasma cutter, by all means use it(same safety precautions apply for galve pipe), but since the question was asked in the first place, I am guessing the OP dosn't have one available