The easiest (and more than likely least dependable) way of addressing your most paramount conundrum would be to take two small strips of some kind of material (metal or polycarb) and drill a screw hole into the ends of each, and put them in at an angle so as to catch the spring as it compresses, but also allow the plunger rod to pass through. I, however, would in no way recommend doing this.
Alternatively, you could always do something like this combined with what CRS260 suggested and find a steel washer of adequate dimensions, using the two strips from my previous suggestion, but I would also not recommend doing this, as it would still be a very iffy way of fixing your problem.
The only way you could properly address your missing piece would be to either get a stock replacement, or make one for your self from a sheet of 1/4" polycarbonate. What I would recommend doing (as you most likely do not have the proper equipment to correctly machine a new part) is using a hole cutter
of an appropriate diameter to encompass the screw hols and guide rail at the bottom of the peace. Then using your rotary tool attempt to machine the round peace into a somewhat comparable replacement, that would have your two screw posts, as well as the bottom guide rail. Then you would have to find the correct placement of the plunger rod guide in the center of your piece and cut that in. Finally, you may want to place a pad of some kind at the back of your new piece to simulate the white rubber washer at the back of the stock piece, as this is more than likely a cushion for the rapidly moving plunger tube, an o-ring may even work.
And ramping up the voltage that much would probably be an utter disaster without a very good solution to your problem.
Or you could just go and buy a new blaster and sell that one off for parts.