Hi Rob,
Making your own trampoline is not a big deal. The problem is not the machine but the thread you should use.

A good thread is Dabond 2000(V92) available from Bainbridge. Just do a search, they are on the net.
For the Trampoline material, have a look in the classified adds of a 'Multihulls' Magazine. Someone(for > 10 years now) is selling extra wide trampoline material, good for one piece diagonal cut trampolines.
Bainbridge also sells the same mesh but narrower(Polypropylene **the real mccoy)

Orrrrrr just call your local canvas guru and get a price for the job. I would build one for ~$350-400US , not sure what Nacra is selling them for these days.

Good luck
Marc