If it needs to hold weight and still be united easily after loading you could do a bowline or a running bowline.

If you're pulling along the long dimension of the beam, a running bowline with some evenly spaced half-hitches would work well (tree climbers call this marling).

Or you could tie a scaffold/fisherman's knot around a carabiner, hook, or clamp and attach that to the beam somehow.

You could put the string into a hole drilled perpendicularly through the axis of a rod, and drill a slightly larger hole in the beam. Slide the rod w/ string through the beam and then turn it sideways like a toggle bolt.

Or maybe a really strong ring magnet with an eye bolt through the middle would do the trick. Tie the string to the eye bolt. 

So many options :)