3/8" of poly fill 1"x1" under pan helps some but will still be set off by shorttailed shrews. 1/2" is better but may possibly not be tripped by smaller ermine(which might be OK). The professional M320 Victor traps have a heavier pan than any other I have found and are resistant to pygmy shrews. Am experimenting with higher holes and anti-climb material on front with good results before I ended season. Not enough time out to judge how it affected ermine refusal yet.
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I've learned enough thru the years to now know that I don't know enough.