There are no fixed rules here. Your aim in helmet outfitting is to pad out a comfortable helmet that is snug to your head. You don’t want lots of wobble, but a helmet that is too tight is just as bad. Below you can see the standard white padding that comes with West Polo Kayak Polo Helmets, but you can cut it down or add extra foam with glue depending on your size requirements.
It can be useful to wipe down the inside of the helmet with acetone or some other solvent to make sure the sticky foam has good adhesion with the carbon or fibreglass. Don’t worry too much if you don’t have access to acetone, just make sure the helmet is clean and free from dust. Even if the helmet outfitting comes loose after some game time, you can easily glue it back it. Carbon helmet outfitting isn’t complicated!
You can use sandpaper to smooth off any edges and to get a nice rounded finish. Scissors and a bread knife are also good tools for shaping foam. Once the helmet is padded, adjusting the straps is a really important task to make sure the helmet stays on your head without wobbling.