Hi Dan,
I tried your solution but unfortunatly it doesnt work. I’ve got still the same issue. I tried also to convert the extrude meshes in blocking volumes and again back to static meshes. I dont know why the fluid ignores the floor when i combine them.
You can look in my other topic to understand the problem just search for Niagara Plugin broke simulation between meshes. There you can find another example with the default niagara fluid system of the contentexample project. I tried also the converting method in this project and my 2 meshes react different as before. Now only 1 have the perfect collision with the Fluid the duplicate mesh will ignore the Fluid completely anywhere the distance between them is little or large.
And the default meshes of the project still work.
Hope you still can help me out 
Thx for your response