Heating PVC pipe and pushing over a mold involves stretching the pipe. Naturally, the pipe wants to spring back to it's previous position. As you move to larger diameter pipes, the force involved eventually becomes too great to overcome with body weight alone.

Using a lever arrangement as shown here will help. It also allows you to hold the flare down whilst using a level to check that you are still 90 degrees to the table. This port is 10 inch diameter with a 40mm flare.

When doing the second flare for a one-piece port, the cross peice makes it possible to use a tape measure to check that your overall length is what you want.

With a large port, getting the entire end hot enough to flare can be a problem. By the time you heat one side, the other side has cooled. This is particularly true if there's any wind about. The trick here is to add a foil shield as shown. This has the added advantage of funelling the air from your heat gun around the port, rather than most of it being wasted.