I'd build one. That way it can be exactly what you want it to be, as well as being able to tie it into the look of your interior. I designed my one and had it made in aluminium. It's obviously not to everyone's taste and probably won't work for you, but it gives an idea of what you can design on your own :
If you'd like to keep the dash looking completely original, you could make up a board to fit under your rear seat that'll house the radio deck and mount your speakers as per Boyscouts manner or make panels that can fit in the rear panels to house some speakers, this way you can preserve what's becoming a hard to find mini locally by not doing any permanent cutting or changing to the shell.