Many of the newer rear intake gas engines almost require a hole in the firewall, as they tend to get too close. I also like to use velocity stacks, so On many of my aiplanes the intake air is from the other side of the firewall. This gives the benefit of good clean air that is not turbulent or overheated from the rear muffler as well.

It would seem a priming tube would work well for you, and a syringe could give an exact, prescribed dose.