I see this a lot. If the gun shop provides a customer with a tool such as a template or Quicken and the customer fills in the blanks, the customer is creating their own trust and the shop is not acting as an attorney. But hey, I'm not an attorney either.
That's not my point. From everything I've gathered, making your own legal documents is not the best way to go. Particularly for something like this that will come under great scrutiny by a govt. agency who is that zealous. In the same vein, I've also gathered that just writing your will down on a piece of paper will probably not hold water, either.
Texas Infidel is right, there are attorneys who specialize in such things. The one in NH charges $6-800 to do an NFA Trust. But i bet it would hold up better than anything you could do yourself.