Not sure if it applies to horse boxes but any new build trailers or trucks for transporting cattle most have a bunded base with a tank to catch any poo or pee that the cattle produce.
There is another option for your body - buy a horsebox trailer with a rotted chassis & transplant that onto your van. Most of the construction already done for you & you could offset the cost of the horsebox by selling the van body.
There is another option for your body - buy a horsebox trailer with a rotted chassis & transplant that onto your van. Most of the construction already done for you & you could offset the cost of the horsebox by selling the van body.