Cheap and effective idea from Sport Avation good for cables such as carb heat or in this case oil cooler shutter. Just drill a 3/16 hole lengthwise through a bolt of sufficient diameter (maybe 1/2?). Drill hole through firewall, add a couple large area washers and locknut. After cable situated fill in any (very minor) voids with your sealant of choice. Done.
Hollow bolts (already drilled) are available in different sizes. Saw one on a firewall with inner diameter of 5/16 or so. Super neat idea for many firewall penetrating applications. I wondered why we don't see that more often.