If you have the woodwork skills, you could build your own, but timber is a helluva price these days. A small arc would be perfect. There are several places supplying them these days.
I've just bought two self-assembly arcs from Bidgiemire who are near Biggar in the Scottish borders. They've been more than helpful and their smallest arc is around £200 - when taking time and effort into consideration, I can't beat that.
Having said that, a pig is happy as long as it has anywhere warm, dry and draught-free.
How, exactly, do you 'accidentally' buy a boar - they hardly slip into the shopping bag by mistake! I'd love to know.