I ordered a custom box and in the meantime picked up a primo double box cutter for $9 just to hammer on and learn on and entertain me in the meantime. After watching several videos it's clear that just like with a pot, you never want to touch the friction surfaces to your skin to avoid oil, sweat, etc. Then in the same videos the guy is cutting using his thumb as a stopper and resting it directly on the soundboard (edge of box) where sound is created during a yelp.
Your thumb shouldn't be that high up on the soundboards to affect where the lid will touch. I only place my thumb about one inch up on the rail for that technique.
X2
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I cut on the left side of box without using my thumb. Lid or paddle between my first and middle finger, turn the box sideways and cutt away.
Don't eat fried foods, tater chips, or pizza before you practice playing. That's the same as drinking coffee or soda and practicing on a mouth call, or duck an goose calls.