You can try Mitee-Grip from http://www.miteebite.com/
mount it to a solid pc of metal then do your profiling.
If I want to make a small sheet metal item, like a pick guard for an electric guitar, how do I clamp down the sheet metal I am going to cut? Should I first drill all the screw holes the pick guard will need and then temporarely screw it to a piece of plywood so I can cut the border? Yesterday I was trying to cut a border on some sheetmetal and was having problems.
Lots of ways to do it...
1) If there is holes, drill them first and use them to hold it down
2) Vacuum (doesn't work for small parts)
3) Double sided tape
4) Hot glue... glue it and melt the glue later to get it off, and soak the part in acetone to get the glue off
5) Low-melt point metal. MSC and other places sell this stuff. It melts at a low temp, you embed your part in it, machine it, then melt it off
6) The mitee bite stuff, as mentioned
7) Depending on the shape of the part, clamp another piece over the top, make a sandwich and machine through the top through your part too at the same time
I used the hot glue method for machining some 0.050" thick copper. I had to route a pretty complex pattern, worked like a charm, they came out perfectly flat and with nice sharp edges.
You can do it like you said, clamp down a piece of plywood, drill the mounting holes, put screws in there to hold the pic-gaurd the best you can, the put screws around the outer perimeter of the pic-gaurd, and try to make your start/stop of the profile opp as close as you can to the mounting screw holding the part.
Would be better to use on of SRT's suggestions though.
Me personally, I would use a bandsaw, if no bandsaw I would use a 4-1/2" angle grinder with a .035" thick cut off wheel and cut it out by hand and finish it on a pedastal grinder. Or if you have access to a plasma cutter, as long as it's not SS, SS tends to warp when cutting manually because you can't get the optimum feeds/speeds and almost always end up using too much heat.
In my experiance milling sheet metal is a good way to break flutes off of expensive tools.