The GATEWAY and EVB both have mounting holes. What was behind the decision to not have mounting holes on the POE?
I feel it could really use them.
We aimed on having as small dimensions as possible and holes take a lot of space, keeping components and planes clear around them also takes space. What is more important is that the ESP32-PoE has two rows of headers at both sides at 0.1" step and these are good enough for solid attachment to other pieces of equipment. Search for matching headers or breadboard-like solutions.