Jon,
OK - if your totally serious about wanting to pursue this then I want you to speak to a bunch of people...
Kirk Krack,
Tanya Streeter,
Aharon & MT Solomons,
Frederic Buyle and
Loic LeFerme are all people who you need to speak to in-depth before you even consider building a sled.
All the above have years, or in some cases over a decade worth of sled experience and will be able to advise you...however don't be surprised if they stipulate you should go on a sled course before attempting any sled diving.
I personally know several of the above and only hope you do speak to them before attempting anything, even with Tec Support.
On a final note, I have spent a lot of time with an up-and-coming No-Limits Freediver here in the UK and one of his biggest comments after doing it for 6 months was saying how he was totally unaware of how far he could push it on a sled and how he would most probably be dead now if he hadn't been training with Loic LeFerme and the whole team of Freedivers out of Nice, France.
Think extremely carefully and seek a tremendous amount of advice before pursuing this without experienced (by that I mean sled experience) backup.
