If you are looking for a fixturing clamp, these in-expensive clamps made from Harbor Freight bar clamps may be just the trick. I am currently using the clamps for both woodworking and welding. There's a number of woodworkers who have already hacked the clamp in order to use it as a fixturing clamp.
One approach is to grind off the "nub" that holds that prevents the sliding jaw from being removed. Once you've done this, you can remove the sliding jaw and insert it through the table. There are some problems with this approach. One is that you have to fiddle around to attach the sliding jaw. Another is that you need enough clearance UNDER your table to get the fixed jaw through the hole. A hole as large enough to fit the bar through is also needed.
The inspiration for these clamps comes from StrongHands Inserta clamp system. Instead of a magnetic point though, I use a 5/8" bolt. It's not as convenient as to set up as a magnetic clamp, but it does allow the clamp to also be mounted on a wood surface as well as a steel surface.
Making the clamps is simple. Cut off the fixed jaw of the clamp, and then just mill a slot in the middle of the bolt, and then widen it enough for the bar to fit in. Then, weld the clamp into the bolt. To mill the slot, I used an angle grinder and cutting wheel. Once I had the slot in place I just widen it with a 1/4" grinding wheel and then chamfer the end of the bar clamp so it will fit in the slot.
To weld it up, I tack the corners in place first then finish the weld.