Nothing is particularly important about how you construct this circuit. I used a perf board and point to point wiring as you can see in the picture. Since this goes in a marine environment, I strongly suggest you conformal coat the board to prevent corrosion. My poor man's conformal coating is simply a few coats of clear acrylic spray paint.
Any project box will do for this, but again because this belongs on a boat, plastic is best.

I've tried to account for various designs of ignition and tachometer, but be warned that I've only tried this on a few engines, with a few vintages of ignition coils. I cannot guarantee that it'll work in all circumstances. The tach I used was a Faria and the tach Bill used was one from Princess Auto.

Feel free to email me if you have any questions or comments.
paul.moir@gmail.com