ThangBom, you can take a string of 10ft length and pin one end down. Now put a chalk on the other end and swing it around and draw a circle with it. Now you have your curved part. You can make it as long as you want.
My 20ft bender is just sitting around not used now I am done with my project. I could sell it to you at a discount if you wanted it. I doubt I will be using it again. Send a PM if you want it.
And honestly, I don't think its worth your time to try and build one. They have a very affordable one on buildyourowngreenhouse.com
The one I got was the mid grade one with a stand for it. You can bend a hoop in about 30 seconds with it.