Years ago I had to work inside a 20’ by 20’ by 7’ coal furnace replacing cracked firebrick.

Even after letting the furnace cool for 48 hours it was still 140 degrees in there.

We bought heavy canvas vests with pockets just bigger than re-freezable ice packs to wear while doing the brick work.
Four pockets in front, six pockets in rear.

These allowed you to work longer, but “fresh” ice packs were actually too cold on your chest skin just underneath the icepacks, so we cut “spacers” from old tire inner tubes to slip beneath the ice packs in their pockets, and then pull the spacers out as the ice pack got less painfully cold.
Ice packs on your back did not seem to bother as much.

The canvas vests were over priced from the commercial supply company, but they would be easy to make at home.