I took some measurements intending to do the same thing. There are a few amps that could physically work there, but you'd have to do some minor fabrication to match the amp's mounting holes to the factory bracket. A couple of strips of metal or strong wood would probably do the trick.
The other issue is length - only the smallest amps are short enough to not extend too far "down" into the footwell. You'd have to tell passengers not to lift their feet up too far
I found a few amps that might work - the aforementioned Blaupunkts among them. Obviously more choices if you go with 2-ch instead of 4.