I put a magpul m-lok forearm on mine. Before you order parts, make certain which of the 2 slide assemblies you have.
Mine was the short one, and the forearm that I ordered was for the long one. It took a lengthening adapter, and a couple hours of fitting. But it's done.
Most of the fitting was for clearance of the adapter nut.
I have the Black Aces quad rail on my 590A1, and strapped a Olight Balder Pro on. It comes with a green lazer and the light is super bright! Sure is one cool tool!