Survivor Customs set about making his own. Here's what he says...
You want a half decent enduro/ motorcross tyre with some decent tread. The length of screw will depend on the tyre tread depth, I reckon you want blocks at least 20mm deep,so if you use a 25mm screws you will need to line it with something 5mm thick at least
The screws I used were sourced from a local bolt and fasteners shop... they are M6 x 25mm Hex head self tapping screws with an 8mm head. These are also known as roofing or tech screws but you don't want the ones with the self drilling tips you want the pointed ones.
UPDATE: Bob in Minnesota (where they know a lot about ice racing) says to remember to tape the joint of the liner to stop it nipping the tube. I'm sure Mike at Survivor would have done that, but forgot to say in this quick how to he kindly wrote for us.
Click to find out more about the Snow Quake race we're promoting with Deus ex Machina and Pirelli in the Italian Alps. G