That's honestly the most difficult part. People always find a way to go crazy with a full tank build on someone who just shouldn't be a tank. However, I can think of an item design that would match this purpose:
Goliath's Presence
Kindlegem +
Warden's Mail
Unique Passive: +100% Champion Size
Item Active: PRESENCE OF GOLIATH Channel for up to 3 seconds to disarm enemies within a (based on champion size) radius. Champions disarmed by this Goliath's Presence are only able to autoattack you, the Goliath. This channel is uninterruptible and can be canceled by the user, you can also move while channeling. 45 second cooldown.
If multiple people cast this simultaneously, they are also considered Goliaths and can be autoattacked. Kind of ruins the purpose of this item if you try that.
The idea is that one of the main distinctions between someone building tanky for the hell of it and someone building tank to accomplish their goals is the fact that tanks actually WANT YOU TO HIT THEM. Well, that's not entirely accurate, but the truth is fairly close. Tanks want to drag the enemies' attention away from their allies. This accomplishes that goal in the least abusable way I can think of while maintaining a simple design.
The channel is designed to force the user to be in a state where they are "casting an ability" which prevents the champion from taking advantage of having an enormous combat presence. The fact that they need to activate it, prevents stealth from being applied since they are constantly in combat while this is in effect. It also prevents the user from autoattacking or using their own abilities. While it could be canceled, you are just wasting the item if you are using this to engage since the enemy could just run away. This also has absolutely no effect on enemy ability-casting since it isn't a silence. It could ironically make it more difficult to chase an enemy.
While the active has many flaws, the fact of the matter is that all the flaws hit other classes the most. Tanks that want to "draw aggro" will still find a good use for this item even with the flaws. However, to make this item fancy, it actually makes you:
乇乂ㄒ尺卂 ㄒ卄丨匚匚
It even gives a significant benefit to making your champion bigger or playing bigger champions.