maybe just have them sign a paper and scan it- then send to a custom decal place?
not saying you should be cheap, but I believe the decals could cost less,
but I haven't done either.
I think the stencil sounds weird. maybe you could find a badass paint shop that could just mask it themselves?
maybe that would cost even more?
I'm just shooting ideas here, you can ignore any of the bad ones....
what about just a plain old paint marker?
sand the clear coat off the tailgate,
have your friends paint marker sign it,
then paint a new clear coat over to make it last.
then that's cool too because it's their actual signature instead of a copy.
The ink in Sharpie Water-Based Paint Markers is archival quality, meaning that they contain no acid, which can eat through paper over time. They are safe to use on paper surfaces without fear of erosion. The Oil-based markers, however, are not archival safe