Thanks for the reply. Great questions and exactly why I'm posting this. I'm not sure what the values should be. The DMG doesn't really present guidelines for making your own traps. What do you think are reasonable values for damge, perception etc.
I should probably include save ends the ongoing damage.
You're welcome.
For attack and damage, choose values off the "building an enemy" charts. I'd recommend 'high damage, Limited Damage Expresions' for a single-shot trap like this (DMG page 184). Alternatively, you can go with a lower damage value and add an extra effect (ongoing damage is a good one), which is what you've done here. Attack bonuses as a monster of the same level and a similar type (in this case a Lurker).
(Note that while many traps will be Lurkers or Artillery, you can still have Controller, Brute, and even Soldier or Skirmisher traps. Pick the role that fits the style of the particular trap.)
For skill DCs I would recommend whichever version of the page 42 charts you're using in your games. As published, as the errata, or as you've modified it to suit your group and tastes. Decide if you want it to be easy, average, or hard for your group and go from there.
Remember to think up a couple of reasonable countermeasures (leaping out of the way, smashing triggers, plugging holes, and blocking gaps are all common countermeasures in the traps I've seen) so you have some guidelines ready when your players surprise you.
And I just assumed that the ongoing damage was Save Ends. It almost always is.
