Is there a way to do a random /set -u expire?

I have tried doing

set %doortimer $rand(120,240)
{ set -u $+ %doortimer %door. bla bla bla }
{ set -u $+ $rand(120,240) %door. bla bla bla }

Both error out /set: invalid parameters

Is this even possible? or am I barking up the wrong tree.

This is part of a revolving door in and out of channel script I am working on where if they leave before %door (which is set to $wildsite) has expired it will ban them. But I am seeing that people are learning that right now its a 2 minute time limit, so I want to throw a random generated -u to make it harder to detect a pattern.