Is it safe to use an aluminium backing plate to install a compass?
Thanks for your question. The short answer is yes. In fact, most compass cards (the part of the compass where the symbols for North, South, West, and East, etc. are printed) are actually fabricated out of aluminum. Aluminum is a paramagnetic material and thus has a weak attraction to the greater magnetic field created by the magnets in the compass.
Steel objects, on the other hand, have a strong attraction, and these must be kept at a distance of at least three feet away, or otherwise compensated for in the variation of the compass. Things like DC wiring, speakers, canned goods, and even eyeglass frames will affect the way the compass reads, so you should take caution to keep them removed. Remember that magnetic objects in proximity to a compass may affect the compass readings differently on different headings, except if the object is directly above or directly below the compass.
Here's hoping that this information is useful to you.