James, it would fit, but it would re-classify it as a SBS, which would require registration and a $200 tax stamp.

with the political climate today I would avoid ANY modifications to the shockwave style guns with just the change in the way they interpret a rule you could find yourself in hot water> I expect this to soon happen with pistol braces for the AR pistols also infact they almost did it in 2021 https://townhall.com/tipsheet/mattvespa/2021/06/17/thousands-flood-atf-with-comments-over-rule-change-could-land-millions-of-america-n2591083

James, yes Sir it would but, would require registration as a short barreled shotgun and payment of a $200 Tax Stamp. A Brace would avoid the hassle (example) : krollcorp.com/sbt-590-sbm4-pistol-stabilizing-brace-kit-for-mossberg-590-shockwave-160.html

Putting a stock on a Shockwave would be considered manufacturing a short-barreled shotgun and necessitate an ATF Form 1. If that is part of the plan, then yes this product will fit.