I first thought that champ was totally ****py since I seen most of the player playing it failing hard. But I had to test it, I have to agree that I failed hard at the beginning but I think I finally found a decent build. With this you will not be squishy and will deal more than decent damages.

Feel free to give advices

Since the power of Renekton is like the Rage in WoW (meaning you gain rage when you hit an enemy) you rly need to improve your attack speed, thats why I've put mainly AS runes. Armor Penetration is a must have to remain useful on tank since no item in the build will increase it.

Doran's Blade

Just an item to start with some hp and allow you to remain on your lane as long as possible, with a health potion it's perfect

Berserker Greaves

I think they are better than Lucidity since they allow you to have a decent attack speed early, having attack speed means you will stack up rage quickly and then deal more damage with your spells

Trinity Force

Probably the most important item of the build trinity force gives you hp, critical chance and attack speed, also give your first shot of Ruthless predator 150% more damage.
This item has to be rushed, its a really really important item.

Sunfire Cape

I think this is one of the only recommended item that is good, perfectly synergize with the ulti and give you armor and HP, nothing more to say :p

Atma's Impaler

I think this item is quite obvious regarding to the item previously bought. It increase your damage for something like 80 regarding to the hp you should have if you bought all previous item, increase your critical chance, just perfect. In addition it's like if your ulti increased you damage too, since it increase your hp.

Stark's Fervor

Attack Speed + Lifesteal, need an explanation ? :p

Last Item

I never had to buy another item, since this build is already quite expansive you will need time to buy all of these items. I think you can chose easily depending on the game, if you find yourself not dealing enough damage you can buy something that boots your damages, or maybe phantom dancer, I have no idea myself. Since I never had the time to test it I will not give more advices about it.

Sunfire cape plus Atma's Impaler give you a nice amount of armor, if your opponent team is mainly composed of AD champ you can easily engage just with Slice and Dice and ulti and play like rly damageeful tank. Otherwise play safe and jump on the weakest enemies, slice and dice allow you to easily rush the enemies hidind behind the tank.

Pros :
High survivability
Can easily jump in and out a teamfight
Has a stun
Decent Damage Output

Cons :
This build doesnt rly improve magic resistance