While planning a new project - entertainment deck using 150x50mm steel beams, I realized the clamp from my welder cannot grip the thin side of the beam. As these beams go between posts I cannot attach to the wall either. Solution is a simple magnetic clamp that is easily moved along to each new joint.

Materials:
Magnetron from a scrapped microwave oven
Scrap steel plate - mine was an off cut of the 150x50mm that i cut a 100mm square from (3mm thick)
Scrap piece of reinforcement rod or a bolt

Tools:
Welder - MIG in my case but a stick would work fine.
Angle grinder with a cutoff, a “flap” disc, and a grinding disc

Step 1: Magnets

Break apart the magnetron and remove the two ring magnets. Also there was a round plate that I kept – vanity really, just gave the finished tool a nicer appearance.

Step 2: Steel Preparation & Welding

Both pieces of steel required a bit of cleaning up first. The rod was quite rusted so that was removed, from the ends at least.
The plate is galvanized. If you are not aware of the issues with gal then probably best to to a bit of research for yourself. What I did was use the flap disc to clean the gal off both sides of the plate. Also when welding set up a fan to move any fumes away from you. And finally weld in an open, naturally ventilated area.

Stand the rod using a welding magnet and tack on opposite sides. Remove the magnet and complete the weld. I then ground the weld back to ensure the magnets were able to sit flush at the bottom. Mark the edge around the magnet. Remove the excess with the cut off disc on the grinder.