Have you checked out the
UK Cat Food List. Although not recommended for Merlin, towards the bottom of the list are a couple dry food options that are lower carb you could feed your other kitty, and it wouldn't be quite so bad if Merlin stole some.
At this point in your journey, you only need to worry about him snacking in the couple hours before shot time if he's in lower numbers. You don't want to shoot a food influenced number if he might be too low to shoot without that food influence. In higher numbers, just shoot!