Occasionally it’s impossible to arrange for clamps in some areas. Since I’m building an aluminum airplane, I can use magnets to hold the parts together. These magnets are neodymium rare earth magnets, and they are quite strong. Mine are 1/16 inch thick by 1/2 inch in diameter. It would be better if they were 1/8 inch thick because once in a while these break.
Be careful handling them because they can cause injury, especially if a stack of them gets close to something. If you’re separating some parts that have been clamped with these, as they separate they’ll lose their clamping force and are free to move around. If there’s anything steel around they’ll jump to it as they fall. This can cause injury if you’re not prepared for it. Additionally, the magnets can lodge and stick in unexpected places. Please use caution when working with them.
I buy mine from K&J Magnetics, Inc., and the ones I buy are the N42 type. They are axially magnetized and have a force of about three pounds, becoming less as the gap between them becomes larger. I’ve found that they work just fine across a stainless steel firewall, and parts slightly greater than 1/8 inch thick are not a problem. They can also be stacked for more force if necessary.