High-frequency emissions love long thin gaps. Just grounding the parts against each other is not enough, you have to close the longer gaps, eg with copper tape

High-frequency emissions love long thin gaps. Just grounding the parts against each other is not enough, you have to close the longer gaps, eg with copper tape with conductive glue (sold in electronics shops) and the surface below must be blank too. If you do not do this the gaps might even function as an antenna and amplify the signal. Also, cables coming outside of your box must be shielded too.