Cool - there seems to be some interest in this. Sounds like it will be worthwhile.
Do Twin Jets use "normal" fin lugs and buckles, or do they use a Jet Fin style attachment? I was only planning to get the bits together for the non Jet Fin type fins - the S hook attachment seems to be the simplest and neatest solution - not sure where you could source the attachment hardware that would be needed to do the Jet Fins. They seem a little more complex and outside of the scope of what I was planning. I'll let this thread run until the weekend, and on Sunday evening I'll put an order in for the number of springs needed, organise all the other bits and pieces, and then once it's all together, post a final cost and payment details.
Planned parts at this stage (might change a little):
2 x 11" stainless steel springs 1/2" OD .062" wire
4 x stainless steel S hooks
2 x approx 10" lengths 5/8" ID clear tubing (might use 1/2" - will try both to see which is best)
4 x approx 1.5" lengths 1/2" ID black tubing
2 x lengths cave line tied off inside spring to prevent overexpansion
Assembly should just consist of trimming the springs to length (if required), sliding the tube on, threading the cave line inside of the springs, crimping the S hooks onto each end of the springs with pliers, tying the cave line off the the S hooks to prevent overstretching the spring, and finally crimping the S hooks onto the buckle lugs of each fin with pliers. I'm a size 12 boot, and I measured my foot in the fin, and don't expect to have to trim the spring - YMMV. The next shortest size spring is only 8", which is a bit useless for most people, so I'll stick to 11" to keep things simple. I've got a website where I can post some detailed instructions and photos of me assembling mine once I get the parts.
Ben