On my DIY cab I foresaw the fact that a flat metal roof would drum and echo noise.*
I was fortunate in that I had a padded, quilted fabric (also fireproof) that had been used in aircraft.
I used contact cement to adhere it to the roof panel before attaching it to the frame, in fact is is permanently sandwiched between the roof and frame.
Proved very effective and played a major role in insulation as well as isolation.
For my floor I used 1/2" foam pads that they sell as exercise mats often in box stores. The material is tough, anti skid and 1/2" is a very effective insulation.
*Large aircraft metal skin areas are major noise transmitters as well as conductors. The panels simply drum from vibration.