Like a lot of woodworkers, I'm using PVC plumbing pipe in a central dust collection system in my shop. One drawback for this type of pipe is that the fittings are not designed for upwards of 1000 CFM air flow, and will introduce significant pressure drop to the system. Additionally, once you get up into 6 inch and bigger sizes, the fittings get expensive.
These factors led to designing and printing my own fittings for the DC system. Blast gates, elbows, and wyes. I tried to design these to minimize pressure drop, and to fit the PVC sewer & drain pipe directly, without additional adapter fittings. The fewer fittings, the better for DC performance.
My dust collector has an 8-inch inlet port. I needed to branch out in two different directions to 6" runs of PVC sewer & drain pipe. An 8 x 6 x 6 Wye from my local Menards store (home center) was $89 US! That's a pretty big chunk compared to the cost of the pipe itself.
Like a double wye fitting I designed, this would not fit the build space on my printer, so I separated it into 2 pieces, printed, and glued together after. The two 6" branches accept 6" S&D pipe directly without any other fittings. The 8" end fits snugly into a length of 8" spiral metal pipe that feeds the dust collector.