Probably a silly comment but I've just bought an RPi4 and an Argon One case. The case comes with a couple of thermal pads but you need to peel the backing from either side to reveal the thermal paste sandwiched in the middle.
Not a silly comment - with the rare event of some warm weather in the Scottish Highlands I was also getting a little thermometer. My solution was to buy a laptop cooling pad from Amazon. I went cheap and spent a massive £5.99
Trust Ziva Laptop Cooling Stand for Laptops up to 16-inch, Black
Trust Ziva Laptop Cooling Stand for Laptops up to 16-inch, Black
www.amazon.co.uk
works really well.