I suspected as much. 5/8" isn't the best choice for speaker building. I'm in Houston, and it's a large enough city to have more specialized sellers. I get mine from a place called Houston Hardwoods which has hardwood lumber, veneered mdf and plywood, plain mdf in several thicknesses up to 1.25", mdf boards, and several kinds of veneer in 2' and 4' widths.