Is there a way to render 3d object on top - just like the 2d UI - and avoid the postprocessing laye?

I need to render 3d object as a top layer - just like the 2d UI does - so it is allways visible.

I use HDRP - and 2018.21f1

The way i do now is I add new objects to another layer, add new camera and set culling to that same layer.
If i do this - it renders fine - but removes the post psrocessing effect from the entire image.

I need the post processing effect on the Main camera - but it must not be on the new 3d object camera...

Does anyone know where to play with the UI layer? are there any settings on this UI layer that makes it render on top no matter what?

If i add 3d object to that UI layer - it disapears..

Hope anyone can help - thanks