You can make almost any shape of cookie cutter that you want. I didn't take any pics of the cookie cutters that I made, but made another one just to show you how easy it is!
Step 1: Cut your cans
With the base removed you can use tin snips or all-purpose scissors to cut a slit up the side of the can and also to remove the top rim of the aluminium can. Cut the removed section into strips about 4cm wide. DO wear gloves when doing the cutting, as the cut edges are sharp.
Once you have a nice straight strip of aluminium, use a Dremel MultiTool and grinding stone to dull the edge.
Alternatively, use 60-grit sandpaper to dull the sharp edge.