My contribution.
This is by no means perfect, as it involves the rather cumbersome crates, so you'll always want it hidden behind walls, but it does provide reliable sparks. There isn't a constant flow, as that seemed a little extreme, but I'd imagine more constancy could be worked in with a little effort.

Instructions for Modification:

• The apparatus is four tiles wide by two tiles high. For ease of description, events will be referred to as in tiles 0;0-3;1.
• To modify the number of shield blasts contained within each crate, modify the Delay parameter, currently set to 15, on events 0;1 and 1;1. Hypothetically 1;1 will represent the number of blasts when the level begins, and 0;1 will be subsequent blasts, but JJ2 isn't precise like that, so don't count on it.
• To change the shield blast to plasma instead of fire, modify the Event parameter, currently set to 17, on events 0;1 and 1;1 to 19. Or, really, any number at all for any event at all, but this is just for shield blasts.
• To change the generation rate of the blasts, modify the Delay parameter of 3;1.
• If you'd rather not use a bomb crate because of its explosion, replace the Event parameter of event 3;0 with 46 (gun crate), 47 (carrot crate), or 48 (1up crate) instead. Just keep in mind that this will introduce objects into the level that won't disappear after a few seconds, and could quickly take up a lot of memory/speed.
• If you see a lot of DON'T USE events, replace event 255 in your jcs.ini with the following:
  Code:

  255=MCE Event                      |+|Area      |MCE    |Event  |Event:8|Delay (secs):5

• The generating fly off (electro-blaster) and disappearing/reappearing stomp blocks represent just one way of making the crates repeatedly open. You may certainly replace them with anything that is more appropriate to your level's needs; please be sure to place event 1;1 at the tile the crate will open at.
• Obviously, since it is a small apparatus, you can put it anywhere you want in the level, many times. Just keep in mind that it can use a lot of sprites and you may start seeing your character and other things disappear, which is a good reason to limit the number of blasts contained in each crate...

Let me know if you have any other questions. I hope this is clear and useful.