I seem to never get a decent seal when I drill out the pop rivets for the end cap and repack my exhaust. The outer can deforms allowing gas to escape in the spaces between the rivets. Its like router cylinder ripples slightly opening up a gap.

I have tried sealent during regiment but it doesn't work.

Another ideas?

I was told that clear silicone sealant works well but if you’re struggling the good quality automotive RTV will work, an engine builder I know uses RTV in place of exhaust manifold gaskets on his dyno.

If you’ve over tightened your end cap and made the exhaust can material ripple then you should spend some time beating it back into round both the can and the end cap, make it a tight slip fit and silicone should seal the gaps, your probably blowing the silicone out

I used clear silicone sealant on mine, the cheapest you can find is the best.

Make a tight-fitting ally hoop/rivet band to go around the end and rivet through.
You could try clamping it all down with a large worm-drive band before riveting?