I think it'd look better with the C rail on it, Ian. Especially if you're going to add an EOTech with EOLAD. It'd look too high if you keep the K rail.
With regards to the Masterkey though, I saw a rail mounted shotgun for sale on Facebook somewhere recently, it's a bit old Vietnam style looking, looks like an M37 Ithaca to me, but you might be able to paint it or make it look more modern somehow.
Here it is:
http://www.facebook.com/groups/282655655106558/permalink/510809482291173/
It's a single shot one and he's after £180 for it =/ Might be worth just forgetting about it.
I've added you to the group but the request is pending admin approval, so you might not be able to view it straight away.