The Ned Kelly armour is indeed a splendid idea, but if I may offer an alternative view on the shield idea proposed earlier...
Last year, at work I was given a number of oversized Velcro patches that were designed as marketing material for the less-than-stellar Medal of Honour: Warfighter that we released. My son thought they were great and so I made him a tactical shield with the opposing Velcro on it so he could swap out the badges:
Anyway, the point being that the slot on the side of the shield is designed to secure the slide of a shotgun, so the operator can hook the gun into the slot and work the pump-action with one hand. Perhaps you could do something similar but to work the slide on a Nerf gun while still providing cover.
This shield is about 16" wide and 24" high, but obviously you could make it any size you require.