I measure the components I'm using for the build and cut out cardboard in the proper sizes. Lay out the pieces, move them around trying different layouts, and once I'm happy with where things will go, I know what size it needs to be. I *HATE* wasted space in a mod and hate even more the Uh Oh of having too little space. lol Really nice wood working right there. Keep us updated as you find your tools.Although I'm totally guessing on the interior dimensions needed for different mods