i'm sure others will pipe in with an answer a bit more to your liking, however my thoughts are as follows
simply put, i wouldnt be willing to risk using a generic free trust for NFA stuff.
there are legal considerations beyond what a typical trust entails surrounding the ownership of these items.
Tahts not saying using a free trust wont get approved, but personally i wouldnt want to risk having some language wrong and you, or one of your other trustee/beneficiary's ending up in trouble down the road because of it.
i like to look at it like buying a gun safe - you wouldnt ask where you can get the cheapest gun safe on the planet and expect it to actually secure anything, barring of course someone you know gifting a reasonable one to you anyway. My trust is the same way - which is why i sought out a local attorney who was well versed in 2a Law and more specifically the applications of the NFA to draft mine for me.
Did i get the most expensive trust i could find? Nope. I paid more than some, but less than others. The good part is if something does go awry i have someone to fall back on, not just some boilerplate thing i downloaded off the internet.
at the very least, silencer shop offers a trust for like $99 i think it is when they run sales. IT seems to come fairly well recommended by folks i've seen who used it. give it a google, i bet you'll find lots of happy customers. If i needed to put one together on a super budget this is probably the route i would take.
good luck!