You can approximate and round to the nearest 1/8th inch. In fact, when I did the prototype, some measurements were only accurate to 1/4". Near the throat, it's best to be as accurate as possible but further out, you can be off quite a bit without any adverse effects.We plan to make a flat-pack kit for the 12π basshorn subwoofer very soon, so that may interest you. It will make assembly a lot easier. The panels are all CNC cut, with better than 1/64th" accuracy.
The cost for the 12π flat-pack kit is going to be about $750.00. That's a steal when you realize that the wood alone costs about $300.00, even at good wholesale prices.