How to setup STACKUP in MWO for heatsinks with cavity?

hello,

i have a structure in MWO with the following layers:

a. top metal (RF lines, GND, vias to bot metal)
b. fr4 substrate (dielectric)
c. bot metal (RF line, GND, vias from top metal)
d. heatsink with cavity (cavity below the RF line of bot metal)

question: how do you setup the stackup for this in MWO? top and bot boundary i set to air...