This is a simple player detector that fits on the bottom & side of a nether portal.

It uses exp orbs that get attracted to the player, activating a tripwire string, which is then detected by an observer. It self resets using the observer signal to trigger a dispensor that lauches "bottle o' enchanting" and whatever trap you design. In this case the trap is only used as an example. Additionally an Etho-clock is used to periodically activate the dispensor incase the exp orbs despawn. I only put a stack of items in, so its period is fast, but I would recommend filling 1 of the hoppers fully.

When the arrow trap triggers, some of the arrows will go through the nether portal. As such if you plan to kill a player, I would recommend another weapon choice.

Admittedly bottle o' enchanting are rare, but I could not find any replacement that would be as fast, reliable or compact.