Previously I made some engineers blocks, useful for holding items at right angles using bits from the toy box. The Lego box is about 65 x 50 x 55 high. To add some weight, flattening some lead sheet to suit the internal space, and wrapped the lead in cling film, job done.
For the Lego magnetic clamps I decided to put magnets into Lego bricks, on a try it and test it basis.
A box of 20 (10 x10 x3 mm) Neodymium magnets (approximately £10) which slide easily down the inside of a brick makes 5 pairs of grips. The magnets were fractionally taller than a brick made snapping the bricks closed was a problem.
The brick was drilled with a 3.5 mm drill, to make a slot through the top of the brick. A second brick placed on top and the magnets pushed up a little bit to centralise the attraction.
The two brick unit can be built into any structure with opposing bricks holding similar magnets.
Using two magnets gives greater grip and will hold 2 x 2mm card firmly. I have not tried other sized bricks, but I guess other lego users can come up with ideas.
Rgds
