You can buy the cast iron clamp jaws. Any time I've seen these used,
they've just been fitted up with good old 2x1 pine.
Suck it and see - it won't cost the earth and I expect it will do you fine.
I imagine hickory would be fine. Just make sure you have a large dimension
in the clamping force plane. For example, given two I-beams of equal
weight, one with narrow flanges and a wide web will be stiffer in the plane
of the web than the one with a narrow web with wide flanges.
I bet you're going to need to make it quite wide to provide a lot of
clamping force at three feet.
You might check out wooden boatbuilding web sites for examples of long-
reach wooden clamps. These are frequently used in clamping planking,
usually shop-built from plywood.
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