We do the wood mallet in a cast iron pan method. We use a "bumper" made from the gaskets on the end of water filter cartridges. We glued 2 of these together to get the thickness we wanted. If you have bigger hazelnuts, you could try a stack of 3 of these gaskets. Here's a link to an old video
I did using this method. We have switched to using a
cast iron skillet to contain the nuts, and only use 1 "bumper" at a time as it's faster. The nuts come out mostly whole or halved, with a few getting broken into smaller pieces.