I'm looking to make or buy a caliper for checking wall thickness on a hollow form with an opening of less than 5/8".I know Fennell has one, but it is for the area just under the opening.I would like one that feels the whole way down.I saw Ellsworth uses spring steel that he bends to his desired thickness and feels with that..I would like to know what else is out there that has worked well for you besides laser and fiber-optic. Thanks Rob