I used a lathe, (I finally broke down and bought one) but, with the exception of the drilling template, this could be done with a drill press and grinder, . The only real advantage to this method is that you don't have a bottom nut with a hole it, if that matters to you.
For a better look, click on images to enlarge.
In this variation, the bolt heads cannot be drilled from behind, so a template is needed to keep the drill bit from wandering. I used a discarded hex head from a grade 8, 1" bolt. I cut a shoulder about 3/16" deep, then dished it out so it would fit the contour of the carriage bolt. The carriage bolt in the vise is ready to drill.

This shows the shorter 1/2" carriage
bolt after the square shoulder has been removed and the shank turned to 1/4". The head has been cut thinner and the cap filed free of grade markings and galvanizing. A parting tool is ready to cut it free.

I flipped the template over onto the carriage bolt head, then drilled the first hole. I put a pin in the drilled hole (red arrow) to keep everything in line, then drilled the second hole.
I clamped the spanner bolt wrench upside down in a vise, then placed the drilled small spanner nut on it. This held it in place while I threaded the shank.
This is the finished set. The larger bolt started out as a 12" long carriage bolt. I removed the shoulder and shaped the head, drilled it, then chucked it up backwards in the lathe. On my lathe there is room behind the jaws for the carriage bolt head. I used a center drill, then drilled & taped for the 1/4" bolt.
Note: the shank on a 1/2" bolt is .45", or about 7/16". Keep the in mind when drilling the handles and brackets.