I don't think you need to stress too much about weatherproofing your DIY enclosure. You can get "weatherproof" boxes off Amazon for ~$20, and as long as you mount it somewhere out of direct exposure (e.g. near your pool controller under an eave) you'll be fine. For example, I built an irrigation control valve for an unrelated project into one of these boxes and mounted it to the back of my garden shed. It even comes with 2 cable glands. https://a.co/d/j7hkyDX

Then you can just run a cable over to a section of your pool's PVC piping (probably just after your pump), drill a small 1/4" hole and stick a thermistor into the flow. Something like this is designed for exactly this purpose: https://a.co/d/994WUHf It's even got an o-ring to seal the hole, you just hold it onto the pipe with a cable tie. Whether you get a Pentair-branded temp sensor or not, they're mostly all 10kOhm thermistors, which you can easily add to ESPHome as an NTC sensor: https://esphome.io/components/sensor/ntc.html

Hope that helps. I'm also a big fan of DIY :)