Those nice round flare ports are pretty expensive, especially genuine Aeroport brand ones (which are the best). So I make my own, and here's a DIY guide to making your own.
I'll assume you know how to use the tools and such. Be safe & sensible.
1x PVC pipe in desired diameter (Eg. 6") and approx. required length
1x Heat gun.
1x Bucket of cool water
1x Metal mixing bowl with rounded bottom (approx. the right shape for a port flare). Make sure its metal, plastic melts!
First get your pipe, lay down the measuring tape and mark it with a pencil.
Because of the flaring, add around 25mm onto the end of the port. EG. If you design needs a 30cm port, mark 32.5cm.
Get out the hacksaw and slice your pipe down. De-burr the pipe with a deburring tool, sand paper or pocket knife. You need to de-burr otherwise the heat will cause it to slag and your port will become a permanent fixture of the bowl.
Now gently heat the end of the pipe. The purpose is to smooth down the edge a little bit, and get some temperature into the pipe before we start working it. You don’t want to be melting the pipe just yet, just getting some temperature into it.
Grab the cold end of the pipe, flip it over and place the warmed end onto the end of the bowl. Now is when you want to seriously warm it up. Heat around the base of the pipe (approx. 1cm) and slowly rotate it whilst doing it. With some practice you'll know how much is enough heat and how quickly to turn the pipe around the bowl.
After a while you'll notice the edge of the pipe is starting to flare of its own accord, so start applying some pressure to the end of the pipe to cause it to flare more.
Now that you've got the port flared out to where you want it, really put pressure behind it. This will open the flare up more than you want, but when the pipe cools it'll contract back to where you want it. Slowly tip cool water down the center of the pipe, you need just enough to cool the PVC down and set it into the flared shape.
Here's the finished product. To clean it up hit it with some 180 grit sand paper around the edges to make it smooth or even out any edges, then wipe it out with a clean cloth. With some primer, you can paint the ports any colour you like.
Do this with a stepped joining pipe end of the appropriate ID and you can have them double ended.
Do it with threaded lengths and you can make screw on flares to test different designs or make variable length ports for tuning.