There used to be a man on eBay who sold the tin curves for around £40 each (brand new) if you put one or two rows of block and the curves on top/outside it would be very secure
You can also get insulated breeze blocks (thermolight or something they're called) if you get cold winters or will ever think about farrowing in the sty it would help (but at about £1 per block, they're not the cheapest - you can saw them to size very nearly with a hand saw though and they're lighter and easier to handle).
It might be worth putting in a concrete pad for the base?
Remember to face the opening south (other directions are colder/wetter) and have a run on the base away from the house - last thing you want is for rain/urine to run into the bedding
Also, look out for old plastic oil tanks, these make great floored arcs for kunekune. You just need to cut a hole for a doorway and wash it out a few times. They're easy to move, robust, being dark green they blend in lovely and you can usually pick second hand ones up for about £30-£40. We use them and find them ideal size for a couple of kunekune or a sow and litter
HTH
Kx