Easiest option is just plain untreated pine
From hardward store. Pine wood in large pieces is not an issue like pine shavings (which are shaved and have more surface area overall). If you then paint it with plastikote it helps seal it a bit and also makes it wipeable. Plastikote water based enamel paint has virtually no odour and is pet safe
A couple of coats helps it to be wipe clean.
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
It comes in different colours too.