There is an argument that the attachment being none too solid is a good thing, as if the get caught in grass or something they will pull out with the minimum of damage. If you are not happy you need to open up the wingtips and fit hardwood blocks to screw into.
Magnets should also works. Again you will need to mount these on hardwood blocks.
Gets some that have good depth and then roughen the back and the circumference with a Permagrit tool. On the rails insert a piece of ply to back them up to. It is really important to get the mating surfaces flat. In the wingtip make a piece of thick ply with a hole in it the size of the magnet and glue one in there the correct depth to match face to face with the one in the tip. Make two to attach the rail and fix them with epoxy filled with cotton fibre. It takes too long for hysol to go off and any way you need minute quantities only.
John