We've had custom comp joints for light duty (yard/barn use) made by Ohio Valley Trackwork and very satisfied with the quality, service, and price! They mill these out of 1" bar stock, milling the top and bottom fishing angles to match the two sizes of rail, and milling the vertical step. The horizontal step to make the gauge faces flush on the gauge side is done by changing the depth of milling on the fishing surfaces of the bar so the rail sits further in on that side. Drilled holes for the bolts with ovals milled on one bar or the other, as specified.

I imagine they could make straight joint bar sets the same way.