SSG's are very simple to fabricate. Below is a picture of what I use in my Marauders. These are all 'carriers' that I can swap out by removing a rear 5/8-18 plug that I had custom made and had knurled by a cool member here. Each one holds a different tune so I don't have to mess with different preloads. These are crudely made but polished where it matters/where sliding motions occur.
The smoothed threaded end of the bolt sits in a small hex plug that sits within the aforementioned 5/8-18 plug, the small hex plug allows adjustment of the free flight distance and keeps the ssg in line with the hammer at all times, and the front of the bolt sits within the hammer, with enough length on both sides that it won't slip out during hammer travel, and remains smooth in operation.
I don't see why this design cannot be adopted by any gun that currently doesn't have an SSG or adjustable freeflight, one would just need a drill, bits, a tap, a rotary tool / method to round/smooth a few sections, and patience. There are many other ways to skin the cat, not saying mine is best, however I use what I currently do because its the best solution I personally came up with.
Couple of nylon washers/bushings, a bolt, and a nut, cost no more than 2$ at a hardware store per unit. I prefer finer threads which allow more precise adjustments, so I use 1/4-28 bolts, however this specific size isn't necessary, but what is, is not having the bolt be fully threaded, you want one that is shouldered so the spring or front bushing/washer doesn't get caught up during cocking or during your shot cycle.
Depending on the gun, you may need a different size of the below combination, from length to width, but most any gun should be able to accommodate such design.
-Matt