Here's mine, thanks to the fitters at work.
3mm mild steel. Google "flat pack fire pit" in images and you'll find plans for it.
Works really well, but you've got to get it really hot before placing any hardwood in it. Once you have just keep "topping" up with some pine before the next bit of hardwood.
Also, use "smaller" blocks of hardwood, if you buy the type in the bag from a servo, best to split them before use
Cheers, Scott