Hi Mick,
Murray ordered from these guys recently:
www.seatexboards.com/product-category/foil-box/Shipping is pretty expensive but he's happy with the result.
I bought one of these from the US - its a bit narrower.
www.kingspaddlesports.com/collections/accessories/products/deep-tuttle-boxDepending on the board you're thinking of using, might be cheaper to buy another old formula board rather than retrofitting.