Andy, On smaller aircraft in 1/72 to 1/48 scale where the thinkness of the plastic is only 1-2 mm I would expect the magnets to hold if they are placed close to the center of gravity.
However, on larger 1/32 scale aircraft I think you have to place the magnets so they are on the outside of the aircraft. If you are careful and place the magnets prior to painting I believe they would be almost invisible. Make sure to check the polarity of the magnets before gluing both sets in place.
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