about to build four MTH-46LC speakers, but it should be a bit more material and i wanted to know if this plan works with 18mm multiplex boards an how the internal and external measurements are calculated. thanks in advance
The MTH-46LC is designed to be build using 18 mm boards. You can use a free program such as Sketchup online, draw the plan and it will tell you the angles you need to cut. Or look if someone has done that for you already.