As long as there’s no jumping or climbing involved, having a small area with no foundations shouldn’t be a problem.

I don’t know if anything changed between the beta and live on this, but when I was messing around with it on the beta client, doors, or gate doors at least, did seem to affect whether the purge would spawn or not. I had a very simple base set up that consisted of nothing but a wall with a gate and the coffer in the middle. There wasn’t even a floor under the coffer. If I had a gate door in place, the purge would not spawn. If I removed it, they spawned.

I did this test several times in both configurations and the results were consistent. I haven’t tried it on live yet to see if I get the same result, but it might be something worth considering.

If nothing else, its a pretty simple test to do just removing the doors from the door frames leading from outside to your coffer.