Personally, I would use any fence waterproofing treatment which says it's safe for plants. Definitely not creosote, but I'm not sure that's available now. You might be interested in this article
I laughed when I saw the advert, though. That's a standard pallet packing frame! See here for a similar one, standing on a pallet. If you google "pallet collar" you may be able to get it cheaper, bearing in mind you're not buying just one.