The innerwebs are littered with mint tin amplifiers. I wanted an earbud practice amp for my (sketchy) guitar playing. I used the Little Gem design on runoffgroove.
I managed to cram everything into this chocolate tin, after drenching the parts in 65% cacao dark chocolate.
Since the battery takes up more room than almost everything else combined, velcro-ing it to the outside allowed me to use a really small enclosure. Inside is a little bit of circuitry, an LED, and a mini headphone jack on top.
The back end just barely fits a 1/4" input jack and toggle switch. I ditched a necessary volume control and picked a resistor that worked well with the output of the guitar.
My soldering skills are just good enough to avoid permanent self-inflicted wounds. The heart of everything is that little black square (the op-amp) in the middle.
You really only get one shot at shoving everything in there. I fully expected it to self-immolate when powered but it worked perfectly.
For a practice amp, it sounds pretty decent. For a practice amp made with $5 worth of parts jammed into a chocolate tin with a Duracell monkey on its back, it sounds fantastic.