How to build polymer beads (e.g. Polystyrene or PMMA) with sulfate group (negative charge) using amorphous cell construction?

I tried to build one PS polymer chain using Build -> homopolymer function, then add sulfate group charge to both ends of a chain. Is there a way to pack 1000 chains (for instance) into a spherical cell or boundary, with sulfate group charge projected outwards? (just like charge particle being synthesized in emulsion polymerization)

The existing cells allowed using amorphous construction are either periodic cell or confined layer... Any better suggestion?