I can't find an air filter that will fit on where my turbo is. The only option I might have is an HKS mushroom filter. I need to know how thick it is from the front to the backplate, not including the flange.

go to k&n website. they will have a filter to fit your app. They have all of their filters listed with all of the specs

I looked there. Unfortunately none of them will fit. I only have a small space to put a filter in.