great helms usually dont have padding or a liner, as they are worn over either a padded bascinet, or over a maille coif with padding underneath..
i have made the same helm, based on the brian price pattern, but i 'raised' the crown, and punched the holes with a whitney punch.
i find making spangenhelms by drilling the holes with roofing screws to be the best option though, as they hold everything together, and when you want to rivet, you just unscrew the screw, and pop in the rivet.
i find it almost impossible to pre drill the plates before shaping, as they will never line up perfectly.
use the short 'rivet' drill bits that have a cutting end on both ends, because they are very hard to break compared to the standard size bit, and you only need to drill through 4mm at the most, so having a short bit makes all the difference.