A padlock should work fine.
My suggestion would be to draw it in the center of the graphic, but I'm mostly just throwing that out there - it might not be any better than the placement now. That said, I think whatever is done should always be displayed, i.e. the player should not have to hover over the exit to distinguish a locked from an unlocked exit, as locked exits/unlocked exits frequently have identical silhouettes, unlike traps, water, fire, etc. Other than that, as long as it communicates it clearly, it should be fine.