It appears no to matter much, but around here (Bloomington, Indiana) the most common rot resistant woods are Black Locust, White Oak and Cedar. I might just wind up using poplar because it is easy to source and cheap. Especially for the wood parts that aren’t in direct contact with the CEBs. Not sure if black Locust everywhere that touches the CEBs is overkill …

Tip seems to think that poplar should be fine, especially if it is coated with pine tar in places it touches the CEBs.