Here is the link of seller, he told me next: "The module is very small so will heat up for short period of time." ...
Don't see a link to the seller in your post. Sounds to me like you went for a cheap price instead of a reliable product that performs to a published specification. Umm... did the seller not provide any specifications? Did you provide the seller with specifications of what you wanted? Sounds to me like you bought a
"pig in a poke" without closely examining the contents of the poke.
... Its price is ~1$ so it makes no sense to pay 50 bux fo cooling,
Since when does the cost of doing anything cheap trump the cost of doing it right? If ~1$ cost doesn't do what you want without overheating and burning out, you just paid for a piece of junk, a "pig in a poke." Order a four dozen more and wait forty weeks to get them. Let us know when you have gone through 50 bux worth of parts.
... im sure there is cheaper way. BTW circut's size is equal to nail.
It appears you may be serious, but not desperate enough yet to do it right. So here is the rock-bottom cheaper way: place the device inside a sealed plastic bag. Press out as much air as you can with your fingers before sealing the bag sufficiently to keep water from entering the bag. Place your
Step down converter with wires attached, now safely sealed in the plastic bag, into a leak-proof box. Surround the device in its plastic bag with crushed ice. As the ice melts from heating up the device, add more ice. From time to time, pour off some of (but not all of) the melted ice water. Wash, rinse, and repeat until finished.
BTW what are the nail dimensions and shape, and what does that have to do with anything? Round nail? Flat nail? Framing nail? Roofing nail? Finger nail? WTF? Maybe there is a translation problem?
