I've done a few over the years and there is a very fine line between 'loosely packed' and 'TOO loosely packed'.
The padding tends to compress over time (a bit like your favourite helmet gets a bit baggier and windier over the years).
If you only put one or two rivets in each end but don't pop them you can then hold the can together with a ratchet strap lengthways and try it for sound a few times before sealing and pop riveting it a final time. It's an easy job on aftermarket cans, it's OEM that are bas***ds.
Good luck experimenting.