Here are a few of those jigs. They are basically 1mm (0.040") piano wire poked into holes drilled in 3mm (1/8") MDF.

The trick is to know where to drill, and accurately drill the holes. Although more low-tech than the 3D printed bending jigs above, they are (probably?) more heat resistant for soldering. And you can see scorch marks on the jigs.
Below is the jig I used for making the Meeker Southern SW9 handrails. They are very different than the MTB railings as used on a lot of SW9's. You really need to know where the handrail mounts to the chassis or body, and the dimensions between these points, and the thickness of wire that you will be using.

A good straight-on photo of what you want to make is also very handy, but you can make an educated guess with an angled photo based on the mounting points and also heights measured against body details. I use a pair of calipers to measure points and to lay out for drilling holes.
You can see the styrene drilling template that I use. You can also see (zoomed in on the Flickr page) score lines on the styrene where I have used the calipers to mark out drilling points. I use a predetermined point below, and to one side of the template to take all my measurements from. Something like 10mm is easy to add onto the measurements you need to transfer. Just make sure the edge of the material is square! The SW9 handrail is quite complicated and so I have penciled in marks where the wire needs to lay on the jig. The smaller holes are where the end of the handrail bends over into holes in the pilot.