I haven't seen anything (not that I've really looked), but it shouldn't be that hard to extrude a cylinder that will slip over the muzzle and add the greebling you'd like.

If you're after an actual, functioning suppressor, if you're in the US, you've got a whole set of laws and the ATF to contend with. I kinda doubt that you'll just find files for that just sitting around thingiverse. If you're not in the US, check with your local firearm regulations.

Frankly, a firearm suppressor (aside from having too small a barrel for Rival) probably wouldn't work that well on a flywheel blaster anyway. The noise is not from the expansion of gasses, but from unbalanced flywheels spinning really fast, and further amplified by the hollow blaster body. You'd probably be better served by filling the dead space in the blaster with foam, epoxy, hot glue, etc to take up space to cut the echos and increase the mass to drop the resonant frequency.

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Also, Rival rail is not picatinny compatible; you'll either need to buy an adaptor rail (like this Worker rail or this adaptor from OOD) or find an stl.

I hope you're referring to an airsoft eotech clone; Rival isn't anywhere near accurate to warrant an actual real steel sight for anything but the memes.