You can also just use Sallet helmets (Mirliton do some, White Knight here on LAF used to offer some, and there's some in the Empire Greatswords kit too).
Mated to SM plastic scouts, you get a bulky but reasonable Arbites.
Alternatively, the Judge Dredd models aren't too far off (although you'll have to be picky about which ones you get), and you can cut the arm/shoulders off to replace them with plastic Space Marine arms for the right look.