It’s all about the kerf adjustment. Which box maker did you use? It may be called a few things, like “kerf” or ”cut width”, if you let us know which one you’re using we can help you figure it out if it’s not obvious.
How much adjustment to use is somewhat dependent on your material and settings. 0.006-0.008” is a good range for “tight” fits for most 1/8” materials but you’d have to test a little to be sure.
Previously: