From what I’ve seen:

Many of the fans seem to be such different sizes, placements, etc. It seems like no one really knows how to cool the steam deck from the back at this time.

I have also never seen someone’s default steam deck getting overheating issues. But I don’t work for valve’s warranty/RMA department.

Personally, I haven’t had any issues playing GTAV on my steam deck directly in my lap. A friend of mine has the JSAUX plastic back plate, their thermal pad doesn’t fit flush, and removing the thermal pad doesn’t affect temperature more or less than the original.

I am of the opinion that no accessory is needed for 1-2 hours of play in terms of cooling, it will be fine for years to come. Battery, however, will be your main issue. I’d get an external battery, and some companies make cases that can hold the battery to the back.

Let me know if you need any help getting your steam deck set up! Or if you need any advice on some things you should do first with your SD.