Good options for modular off the shelf options has been posted above by the others, so I'll address more of the DIY fabrication aspects.
Some SDCs are super lightweight, some are pretty heavy. That in combination with the spacing distance you wish to achieve are the primary constraints.
Steel can quickly make for excess weight aloft, so consider aluminum if using bar-stock, or going with tubing (round, square, whatever) if you decide to fabricate something yourself. Variable bar length can be useful to stay narrow when you need to do so, and go wide when the situation allows it. Nesting or screw-together pole sections can achieve that by adding sections to increase spacing. Think tent poles. Fatter tubing will be stiffer and able to support more weight. One member here uses very thin sectional carbon fiber tubes (think kite struts) as both spacing bar and vertical support pole for miniature mics, getting them 3' apart and something like 15' or so up in a quite visually discreet arrangement allowing use at the front of the audience without huge visual impact. I've looked into both large and small diameter carbon fiber tubes and connectors in the past and far more options are available now than just a few years ago. Different fabrication skills of bonding rather than metal working are required there. I repurpose black set-top telescopic TV aerials to support lightweight microphones which I started doing a decade ago and have yet to find a better solution as they provide variable spacing (coincident out to about 2 meters), quickly retract into into a very compact format for transport, have no loose parts to loose, and are nearly invisible. I can keep things tight, then quickly extend out to 5' or whatever once the lights go down. The arrangement allows me to run stealthier and record using wide-spaced configurations in situations were I otherwise would not be able to do so.
These kind of setups may or may not be appropriate for you, yet hopefully provide some ideas to run with.
[Edit- To make the telescopic antennas work, I fabricate parts out of aluminum bar-stock to which the antennas with integral hinges are mechanically fastened, which fit the stud on a stand, clamp or other mount]