Why actually f-hole? It is obvious. The sound hole has the same shape as the small F. So that a Cigarbox guitar can reproduce its full sound so-called sound holes are needed. In a pure electric guitar can be dispensed with because the sound is transmitted purely by the string vibration on the pickup. With acoustic guitars a sound hole is inevitable. As sound holes for Cigarbox guitars are suitable, for example, pebbles attachment rings. But it can also be used a drain screen of a shower tray. Here you can let your imagination run free. You can also use what you like
For older instruments, C holes were used. Well-known is the so-called F-hole that is used in all string instruments. The F-holes are placed to the left and right of the bridge.