I also have one of the soldering iron things, bought it with a 50% off coupon, think I spent about $12. It works well. You do need to make the lines thick enough, it was also enough that I could use a Pounce pad with it.
Usually I end up making a design on parchment paper I get from the dollar store ($1 for a 1x25' roll) and sewing through it. There are some downsides to this technique, but with my vision issues it is currently the best answer I have for when I want something more complicated. We had a thread recently discussing it, I'll do a bit of a look for it.
Edit: Here's that thread
Custom Quilting on Domestic Machine