Fit a suitable size of flat bar in each end, I suppose s/steel for your app. The size of bar can be found by trial and error, there is somewhere a formula for doing this but can't find it, sorry. This is the usual way to do this PS use smooth vice jaws.

I formed the ends on some mudgaurd stays years ago and probably still have the formers. It is necessary to form a rounded end to the main tube not a squared off one , as would be obtained simply by squeezing between the vice jaws. If I remember. I made two half formers by driiling down a length of square bar, which was then sawn down in half. I then used a ball ended cutter to form the hemi spherical ends. Then the cavities were extended with a milling cutter to accomodate the flattened end. It is best tor remove all sharp corners slightly with a file. A heavy vice was used to apply the pressure. I remember that it did an entirely neat and satisfactory job.

I see the job is now done, but just for reference, I had to re-bend a mooring hook in 1/2" stainless steel. Heated to a red heat I was able to bend it by hand with one end in the bench vice.

Neil