As mentioned they are for seat posts, unless your "tubing" is same diameter as the seatpost they are designed for, If you do prise them open, then you have a problem of needing longer screws to bridge the gap on the clamps if the "tubing" is far bigger than the size they are designed for. Plus you will also have the problem of the "faces" of the clamp not being parallel possibly, so trying to get a screw to fit in both sides of the clamp to engage the thread would be difficult if not impossible without maybe damaging the thread.
I fitted two holders on a frame, and I got two sets of clamps to do it from my LBS, which could be easily made if needed. They were a long strip of very thin flexible metal, with a hole drilled at both ends which were bent outwards 90 degrees which a longish nut and bolt would pass through, which wrapped around the tubing and also held the holder. If I had time I could take a pic and post it, but a bit busy at moment