I’m surprised that in 5e where this appeared nobody noted that Falling Damage applies to characters falling, and maxes out at 20d6 (PHB). There is no specific height by d6 rule for being hit by falling objects, only some guidelines about improvising damage dice from environmental things based on how deadly it should be.
As usual when this comes up, I have to point out that the entire premise is Homebrew - even if Falling Damage rules applied to Thrown Objects (they do not), it would max at 20d6.
The correct ruling is it is an Thrown Improvised Weapon and does probably 1d6 (plus Dex, 0 for a commoner, iirc) - and requires a non-proficiency attack roll to hit.
Why? Because DnD is not a physics simulator. If you want the rules to apply at all, it’s just a thrown object no matter how far it got handed on first. If you want to Homebrew rules about things traveling distances adding momentum, and hence damage, that has nothing to do with RAW.
Source: Forever DM