It’s funny, I was thinking about posting a topic about a “device” I recently “invented” to help me develop a proper DSX motion when I saw this topic 
Here’s how it looks.

The piece of plastic sheet is inserted between the strings so that only one string (the 3rd at the moment) goes over, while the rest of strings are covered. And the piece of foam behind the strings helps to keep the plastic in place and, in the same time, works as a damper decreasing sound the piece of plastic produces during tremolo-picking the available string. (It clicks awfully loud.)
I’m yet to see if this thing is really helpful in developing picking skills. So far, it lets me achieve fast tremolo-picking tempos in order to try different hand positions and pick grips and find which is the most efficient for me. I couldn’t get to high enough tempos without it because I’m absolute beginner and my right hand can’t keep the same pick depth which totally ruins the smoothness of motion.
Last but not least, the idea to limit picking depth by putting something behind the strings is not mine, I’ve seen it in one of Troy’s videos for which I’m very grateful.