Compass compensation

[rapidshot]

for my single engine, i bought a new compass without internal compensation and mounted it on top of the panel. actually it is silva 70 un and the deviation is about 20 degrees, is there a way of compensating the compass by the help of external magnets and/or ferrite materials, if this is possible how many pieces do i need and which side is the most effective to fix them

regards

[Mach E Avelli]

If the deviation is a fairly constant 20 degrees on all headings, why not simply move it in its mounting?
If the deviation is wildly different on various headings, you may have a problem if it has no in-built compensation. So all you can do is swing it against a reliable hand bearing compass and construct a deviation card.