12/0/18 taking stoneborn pact. You're double dipping in the benefits of stoneborn because the bonus hp affects cinderhulk and in turn, your ult.

Take 9x AS reds, 9x armor, 9x scaling MR and for quints I would choose between AD, AS, or scaling CDR. I personally take AD.

For items, start talisman and either refillable or hp pot+ control ward. Cho'Gath's clear is actually so healthy that you don't even really need to use potions except to be cautious of invades or ganking.

Start E, max E. The damage you get from it is actually insane and make your clears VERY fast. Max Q second since it's your only source of hard CC and helps a lot when it's on a lower cooldown.

Your build path is pretty flexible but core items are Chilling smite cinderhulk, Righteous Glory, and Gargoyle Stoneplate.

Boots can either be merc treads or ninja tabi, but you'll probably find yourself needing tabi more. Tabi is also typically more helpful than merc treads early on.

After that, items that give cdr like visage and frozen heart are good but just build whatever tank items you need.

I should probably point out that you do NOT need any damage items whatsoever, no matter how far ahead you are. Your damage and effectiveness on Cho'Gath comes from how tanky you are.