Ok, so right when Trundle was released I bought this nice champion, and let me say, he was a disappointment to me (in the beginning), I mostly tried to play him as a DPS champions, and let's just say, it didn't went well :P, so for a while I didn't play Trundle. About two weeks ago I thought I wanted to try Trundle again, and really build him as a hardcore tank, went very well, just try this build and you will see ;).

Pros
Decompose + Regrowth Pendant gives him a perfect boost early game, making him able to recover in a matter of seconds!

A good champion in teamfights

Hard to escape from

Good escape abilities

Cons
His attacks are very limited, and does not do a lot of damage.

His Contaminate does not work on allies :(

Regrowth Pendant
Ok, so in my eyes, this is a very important item early game, because it allows your champion to recover a lot faster.
Giant's Belt
This item provides a good health boost early game, and can help you survive in a variety of situations.
Ionian Boots of Lucidity
These boots are by far my favorite of all boots, they provide a slight cooldown reduction which be needed in many situations, from early game to endgame, these boots could end up saving you or an ally's life. Ionian Boots of Lucidity over other boots, as an example I didn't feel like using Mercury's Treads, because Contaminate can provide the same stats, plus some more.
Warmog's Armor
This is a great item, it provides a very solid health boost. At this point you should be able to tank towers, assuming you got the right amount of team mates with you ;).
Frozen Mallet
This item is great against runner or what ever you would like to call it, when you're fighting with your team mates, this can always help them catching up to your enemy :)
Force of Nature
This item is almost a must, and in general, just a wonder. This is an awesome item endgame, and can help you recover from zero to 100% in less than a couple minutes!
Madred's Bloodrazor
This is where you choose if you want some damage or you wanna go straight tanking withh Warmog's Armor instead of Madred's Bloodrazer.
I really love this item, because it can end up doing a lot of damage to other tanks. Also when you got Contaminate you can get a pretty good attack speed with it, but the choice is yours.
Warmog's Armor
Lastly we got another Warmog's Armor, which you will rarely get, but when you do, it gives a nice health boost endgame