The suggestion of Lee Valley tools is a good one. They stock a large range of rare-earth magnets, as well as magnet cups and matching washers.

I put a pair of 1/16 x 1/4 rare earths into cups that were in turn epoxied into the bottom of my cockpit/hatch, and then screwed the washers to blocks at the aft end of the fuselage compartment. You can turn the fuselage upside down and give it as hard a shake as you want and it doesn't move. In fact, as has already been pointed out, it can be a problem to get the hatch open and I'm thinking of putting a "hard point" into the underside of the cockpit opening so I can pull it up from there without breaking the balsa skin