I tried making my own last spring without much luck. I also tried some NiTi leader material but it went limp below 32*. I finally broke down and purchased the Clam version which came with two springs and was cheaper than the Frabill. The Clam's also went limp in the cold and failed terribly.
The rest of the story - I emailed Clam about the problem. They promptly responded, informing me that they had a material issue that they corrected. The person I talked to sent me a new set of spring bobber's that have been working great. I love the no-kink properties of NiTi.
Hats off to Clam for their customer service in this matter.
I'm still interested in making my own tho if someone were to find some NiTi material that works.