Lorewise maruts are damn near incorruptable, but even just a rogue modron could be cool if done right. A marut is also unlikely to misinterpret the contract, but the signatories might have.

“[…] A marut enforces what is written, not what was meant by or supposed to be understood from the writing. The Kolyarut rejects contracts that contain vague, contradictory, or unenforceable terms. Beyond that, it doesn’t care whether both parties understand what they’re agreeing to.” There’s also a reason they’re called “nigh-unstoppable inevitables”.

I guess now that I think about it, a simulacrum is a construct as well, but I’d have a hard time counting one of them. Most powerful constructs tend to be golems or a colossus like the Walking Statues of Waterdeep, not independent thinkers, just powerful brute force minions.