Never tried a plastic cap, just as long as it doesn't pop off it should be alright. Doesn't hurt to try, I just know which one worked so I used it from then on out. Let me know how the plastic one works out.

If you're gonna use it on a j, you could either wipe the paper in it or roll the bud you are gonna roll in it first. Its pretty sticky, but the bud should soak it up and look nice and wet once you're done.