When I trimmed up all my .357 Herrett, I made up a "trim die" by drilling a hole through a piece of hardwood the outside diameter of the neck of the 30-30 case, then used a drill with the diameter of the body of the case and drilled it deep enough so the case sat with the rim flush on the backside of the block. I then cut this block to a length slightly longer than what I needed.
I used it more as a guide than an actual trim die, but then cut the excess off using a fine toothed hacksaw. This put me in the general neighborhood for length. I finished it off using a power adapter in my trimmer. I was able to do 100 cases in a couple evenings of work without spending any additional money on a trim die, and in my case that was a good tradeoff.