Tactical Airvests seems cool

Now found this by the offchance but idea seems pretty good, basically like the mesh some of us have on our seats, that actually does help cooling the arse, well maybe this could help cool the body, ready for the usual million pound wicking mob to arrive.

https://tactiforce.com/produits/en/airvest/airvest-p228/

We use something very similar under body armour at work. It's quite bulky and does a good job of helping keep cool when it's fairly warm, but if it gets properly hot doesn't do a lot except add to the bulk.