Brush with no collision?

I believe you are looking for Func_brush

It is a generic, parent-able replacement for func_wall, func_illusionary and func_wall_toggle.

https://developer.valvesoftware.com/wiki/Func_brush

Tomm is correct. Use a func_brush and set it's solidity to be "never solid"

This brush will still cast a shadow unless told otherwise.