I used aluminum coil flashing to cover my telescoping tops.
Cutting is quite easy, neatly folding it was much more of a challenge than I would have expected.
To cut, simple use a good straight edge and score (sp?) it with a razor knife by making a few good passes, then fold the sheet away from the side you've marked.
To bend it properly, you really need a metal brake....There are quite a few of these available for reasonable money. However, I didn't have one and decided to give it a shot without it. I just used a straight edge and did a lot of repetative tapping with a rubber maul and tapped the folded corners into place.
It worked, but certainly wasn't pretty or finished looking. I used a thin strip of wood tacked over the seem on the sides to give it a bit more of a finished look.