Trench bowls are made by scoping out the insides like a pumpkin. Sounds simple but in reality it can be quite a challenge. Woodcarvers achieve this by using chisels by making repeated gouges until the desired depth and shape is achieved. That is simply not humanly possible for me to do with my reclaimed southern yellow long leaf pine that was mulled over 200 years ago and is well on its way to becoming stone!
My special sauce is to de-bulk the main channel in the middle with a Ryobi router and then complete the final shape using a Saburrtooth grinding bit on my drill press. If you do deduce to try my method please use extreme care and caution while using the Saburrtooth bit!!! Always wear gloves and still expect to be missing some skin and or be bruised before your project is over!