This part was confusing. The Instruction manual lacked a lot of details about how to do this. It doesn’t mention if either of these parts need dimpling or what size rivets to use. So I posted the Sling Builders Facebook group and they saved the day. Thank you thank you. The preferred solution is to ” dimple the whole sandwich” . And use 4.0 countersunk rivets. And that’s what I did. When I hold the cowling itself up to the plane it still feels like the cowling won’t be as thick as a space that I created. So I’m not sure why that spacer is necessary but it’s in there now. Thanks for your help everybody.