This is a corner piece I've designed to make for example a cube. There are pockets in the back of each tube where magnets can be glued. I use neodymium, diameter 3.5mm length 3mm, which are easy and cheap to buy online. Then sticks, such as aluminium or plastic with similar magnets glued inside or simply steel rods, can be clicked inside.
The inner diameter of each tube is 7mm (which gave me enough margin to fit a 6mm rod). The pocket at the end has a diameter of 4mm. The outer diameter of the tube is 10mm, length is 25mm.