Thank you all for the many suggestions you provided for an enclosure for a mini portable guitar amp, I have plenty of material for future builds. As it turned out the choice was made for me!
For Christmas my son was given a nice guitar-themed embossed leather wallet and that came in a small tin, the size of an old 2oz tobacco tin, which also had a amp-themed design. He asked me to put his mini amp in that and so that's what I did.
As you can see from the pictures It turned out OK..ish. What's important is he likes it
To play over the speaker one removes the lid, to play over headphones one can leave the lid in place. The toggle switch is used to select which. It also has a gain knob to give a more gritty sound, while volume is controlled via the input source. I have also been using it to listen to my iPod, hence the little 6.3mm to 3.5mm adaptor shown in the pictures.
Nick