If the cigar box is big enough and the bridge well positioned on the surface of the box then the guitar will be loud enough to play acoustically for small groups. The bridge needs to be at the rear of the cigar box, but not too close to the edge of the box or the box surface won’t vibrate much and will sound quiet if played acoustically.
However they sound very good and are louder through an amplifier. Since DIY is my obsession I build my own. They are all “practice amps” which is to say of low output power, but are all quite loud for my uses (learning to play).
ADDENDUM: Star grounding is an important technique to get the best sound from an amp. All the several ground connections of the input, power supply, and output sections of the amp are grouped together separately and then joined to the power supply ground which is pin #4. Here is the schematic for the Smokey amp with all grounds shown (It is common not to show the input/output grounds).This (above) shows the three groups of grounds in the circuit.
Below is a diagram of star grounding for this circuit.
NOTE: THESE CIRCUITS ARE NOT COMPLICATED, BUT BUILDING THEM MAY INVOLVE SOME TIME AND SOME FRUSTRATION. IF THE CONSTRUCTION IS NOT GOING WELL, THEN PUT IT ALL DOWN AND COME BACK ANOTHER DAY. USUALLY PROBLEMS INVOLVE POOR SOLDER JOINTS, SHORT CIRCUITS OR FAILURE TO GROUND. TAKE YOUR TIME AND REMEMBER IT IS A HOBBY.
First of all, the Smokey amp, named because the commercial model is built into a rigid cigaret box. This is the easiest, quickest guitar amp I have built: an LM386, 3 capacitors, and one resistor. It runs on 9 or 12 VDC and sounds good through my 8 inch speaker and is loud enough for use in small rooms (practice amp again). The board is from Adafruit Industries.
When I first built this years ago it did not work. It needs a 100 uF electrolytic capacitor between pin 6 and ground.
If one plays the strings lightly then the tone is clear. If one plucks the strings harder there is a little dirt (distortion).
I put a 100 uF capacitor between the power supply and pin #6 (the positive lead of the capacitor placed right at the pin) and added another 47 uF capacitor in parallel with the output capacitor for a little more bass response and a 2.3K resistor between pins #1 and 8 to limit gain.
Here is the “Little Gem” from www.runoffgroove.com. This is based on an LM386 amplifier chip and runs on 9-12 VDC. It has a volume and gain control and plays clean or gritty.
This is the schematic which I altered to improve it for my build: I removed the output 25 Ohm volume control and used a standard 10kOhm potentiometer at the input between the 0.01 capacitor and pin 2 on the LM386. I added a 0.1 uF ceramic capacitor between ground [correction of text] and pin 6 (i.e., in parallel with the 100 uF capacitor). Finally and very important in my case I did not leave pin 7 open, but attached a 10 uF capacitor between the pin and ground. (the negative side of the cap goes to ground). This latter addition removed instability (0scillation) at high gain and high volume.
Possible upgrades include the addition of a buffer (current amp of high input impedance) before the LM386 circuit: “Little GemII” or Ruby amps are examples from the same website.
The ultimate upgrade is a good size speaker. They sound OK, but somewhat tinny, through a 3 inch speaker, but very good through a Celestion 8 inch speaker, for example.