I found this on the Dragon Fruit website. Maybe you could adapt it for a pot. You would probally need a 25 gallon pot and could use a large bamboo pole or PVC wrapped in moss or cork for lighter weight. There are those fiber moss stick for ivy if you could put several together. An old bicyle spoke wheel for the top. You could cut out some of the spokes. I've been thinking about it also. Right now I have mine in a hanging basket and letting it hang down. But pretty soon the pot is going to burst.

Here is a link that might be useful: scan down half way. Dragon fruit tree.

There are many ways to create a support for the dragonfruit. The Thai and Vietnames style of support with the pole in the middle and round ring on top seems to be the most common and decorative. I had used on old tree stump but moving it is hard and tricky. I don't think these ways are very easy to move in and out of the house though. I once seen a very unique and I thought brilliant way of supporting dragonfruit that I think is best suited for pots you plan to move in and out of the house, it was a large tomato cage. It will allow the branch to hang on different levels for fruit set. You could then keep it at a smaller height. Its light. Because it kind of surrounds all the branches it would hold much better without them breaking here and there when you move it. Just use some kind of pole or stake in the middle.