Depends whereabouts u are, Zoe ... and what sort of shops are nearby.
First off, I went to sheet metalworker to have some 2mm thick sheet steel bent around the AKSA heatsinks (which form the sides of the 'box'), then, for the top, a 3mm piece of steel screws down into the top of the heatsinks. I had all that powder coated.
But I also find, 19" 2RU or 3RU rack enclosures go well - which you can buy from electronics components shops - although they don't have a very thick base (which is needed for screwing the toroid bolts down hard).
Regards,
Andy