Is there a tool that will make a rounded cut in flat metal wire?
As you're only working in thin aluminium, I suggest a gouge chisel.  It will need sharpening fairly often (a diamond file is probably easiest) to go through easily but should do a good job.  Use scrapwood clamped down securely as your anvil.  

I don't have a gouge, but I've tested a straight chisel successfully. My scrap aluminium is 3mm thick and a cut of about 8mm long took a couple of blows with a carpenter's mallet on the back of my roughest (not very sharp, as well as chipped) chisel.  This is bigger than your cut, so you should be able to do it in a single blow.  An offcut of oak worktop was a much better anvil than softwood: the cut was easier and the burr on the back much less. If doing a lot you may wish to make a jig, or at least attach a fence/length guide.  

The heavy-duty nail clippers you were thinking of do exist, but I suspect that they'd push the strip out when you squeeze them.

There are also curved tin snips, but ones as curved as this old pair are rare nowadays.