Ka-band LNA module design

A Ka－band LNA(about 40GHz) is in a package of a metal box made of Cu,
I have learnt from a book that the width of the inside box should be less than λ/2 ，or it will lead to waveguide propagation and result in oscillation。So the width is limited within 7.5mm,which seems too narrow for other bias circuits ' location
Can anyone give me some notes?

You can use a bigger metal box. There are many tricks to avoid the resonance.
You can analyse some LNB ( Ku band).
I learned a lot of things from them.

Run a quick simulation in hfss to check out the cavity resonance will give you better understanding about it...