I don't suppose that you have a sheet metal shear available to you so my next choice would be a set of aviation snips.
I was introduced to these through aircraft maintenance, my job for a number of years, prior to that I used to just use regular tin snips. The aviation snips have a mechanical advantage over the tin snips and the jaws are serrated so the snips won't slip when you cut the material. They are usually sold for the type of cut path you intend to do with them, straight, right, or left. Don't waste your money getting a straight set as the only difference between them and one of the right/left pair, will be the color of the handle (yellow) where as the left (red) and right (green) set have the jaws as a mirror image of each other. The left/right ones can also be used to cut straight so no need to have a third pair.
I have now purchased a number of pairs of these both for work and use at home. The last set I bought was the offset ones which put the handles at an angle to the jaws and allows you to cut without having the cut material pass around your hands, these are by far my favorite set and as it turned out everyone in the shop where I worked too. The ones in the picture above are the offset style.
Sears sells these, thats where I got the picture from, along with a lot of hardware stores sometimes sold individually or in sets. Get yourself the set you will use them both. Remember you get what you pay for and the cheep ones usually wear out fast and have the serrations on the jaws cut too deep which leaves the final cut part with a very jagged edge.
