Sorry mate, I have no other knowledge about the seat, I found it when searching e-bay for trials bits, but had logged onto USA site by mistake.
I can sadly admit that I use a bag of straw [from pet shop] and some bungy straps & tank tape as a seat when I convert the trials bike to teach my kids how to ride a motorbike. its not pretty, but it worked!
I guess that you will get laughed at with a straw seat, but as the only trials bike at the enduro they may laugh anyway. send a picture of the finished item, I may copy it as my kids are embarassed by the straw.
Ha! too funny. You Brits