Best material to make cardboard more durable
For your objective, spill-resistant and ding resistant, you're dealing with two parameters that could be combined. As noted in Bill Horvath's answer, the weight will be a factor.
Cardboard for concrete forms is pretty durable and somewhat moisture resistant &quot;out of the box&quot;, and could take mild abuse without modification.
Virtually any paint will protect the material from moisture once cured. The biggest enemy of cardboard is water and other liquids. Paint will be lightweight, the lightest option, and likely the least expensive route. Even rattle-can paint can accomplish water protection and allow for artistic expression as well.
One step up from paint, providing protection and a minimal amount of ding suppression, would be a coating of water-resistant glue. I used &quot;water-resistant&quot;, but some products explicitly state waterproof. One such product is Titebond III Ultimate Wood Glue, which uses those terms on the website. One would brush on several coats, allowing the previous coat to become tacky to avoid drips, runs, and spatters. Once fully cured, the glue becomes a force-distribution surface, spreading out any impacts. If the glue becomes damaged, it's an easy repair.
Other surface finishes which provide for water-repelling features would work, of course. Some finishes may not work for impact protection, while others (epoxy-based products) may be overkill.
Beyond this level, one will most certainly be adding weight. It's not absurd (but pretty close) to consider that a single layer of fiberglass with polyester resin could make a substantially strong cat hoop, but the weight comes into play again.
Polyester resin or epoxy resin of a thin consistency will provide waterproofing and not all that much weight while giving the durability you require. Additionally, using such a coating on the outside will improve the durability of the bearing contact areas. If applied on the inside surface, adding grit prior to curing gives the felines more confidence and less cartoon-cat skittering.