I love this. A Pair & A Spare taught me how to do it.
You will need a sheet of aluminium flashing (found in the roof section of your hardware store), a hammer, scissors and some wire, rope or thick string.
1 Fold over the sharp edge and flatten with a hammer
2 Cut to desired thickness, plus a little extra for folding (this will ruin your scissors)
3 Fold and flatten with a hammer
4 Fold along the short edges and secure with adjustable wire