if roots grow downwards....

can i just get s tube 3 feet long and 4 inches in diameter. like a duct pipe and grow in it?

yup

If you are new to growing I would recommend more conventional approaches.

The roots do more than just grow vertically. They have a significant amount of lateral growth. I have had some plants send out roots that were just under the surface of the soil and ran right to the edge of the 3 gallon pot. You can buy nursery pots at Home Depot or Kmart for two bucks. Keep it simple. There is no point in trying to re-invent the wheel. The one we have works well and will continue to.

i have some growing in pots. maybe i will try to experiment with it after a few batches of successful grow.